I want to make an anti-unwelcome-visitors.... That can be for 2-footed but also dogs or goats: I have hens, guinea pigs and tasty leaves in a dry place!
I have been looking for species that are adapted to my climate, and here is what I have found....
Of course I am lookibg for more ideas!
Please add some!

Ho, I prefer that they have other qualities than only spiny...

My main choice are dovyalis species, as they are thick, not too tall, really spiny, and bear little orange fruits: good but very acid. They were fine cooked with apple and sugar, they give a good taste.
It is also called kei apple, or tropical apricot, but the size is of a cherry.
I plant then 2m appart.

Then there is the natal plum, carissa, that needs more water and grows slowly. I plant them 1m appart. I have the 1st flowers.

I also have 2 zyzyphus jujuba. They also grow slowly.

Prickly pears can do, but they are not so impenetrable, and pads can be broken easily.

I cannot have robinia psseudo acacia, but what acacia could do?
This is difficult to look for, as there are many species, and I have not found any list with a useful classification.
Do you know about such list?
Something like "spiny acacias". Then I would have to match-cross with a list of "drought rresistant acacias".
My climate is dry, frost-free, but not very hot, which make some desert varieties unsuitable.

Have you looked At Osage Orange. They trees were used as waddles to keep in cattle in the west. very drought resistant.

It's just a little too late for fotos due to darkness but I'll add some tomo if I remember.

I don't know which sort of prickly pears you have but mine are absolutely impenetrable unless you are armed with a long heavy cutting tool or a truck.
I've been growing spiny bamboo for a few years, it's very hardy but it doesn't grow fast enough to be useful on my island, maybe OK for you as you get more rain?
Yucca trees are great, I've got a row of alternating tall and short trees in the garden planted before we moved in so not sure about time to establish a hedge, but very cheap to buy in and can often find them in the rubbish having been chopped back, lopped tops root very easily.
Blackberries are growing really fast and fruiting well here, something to add in the mix but possibly would attract rather than deter 2 legged thieves.

It's actually raining here right now, seems the whole island is cloudy today and in the south the coastal regions not wet but a few hundred metres up it's a light shower. You got rain? So much nicer than the high 30s/low 40s we got a couple of weeks ago.

I planted 3 seeds of acacia polyacantha mid april. It was still quite cold getting down below 10C some nights and only one germinated. Here it is about 4 weeks after planting seed.

Got it from an ebay seller I have used a few times. http://www.ebay.co.uk/itm/370825381655

Definitely the Kei Apple, man even elephants and lions respect those spines.

I knew about osage orange, but did not know it was spiny, so I will have a look at their necessities.
I think they are not edible fruits?

Yes the kei apple is my best, but don't want to plant tens and tens of them!

Steve, yes we had this surprising rain after hot calima from the sahara! Overall a quite dry winter... (sigh....)
I am quite surprise that you say blackberries fruit, because here they mostly leave and leave! Same with raspberry: I am looking for seeds of the mysore raspberry, from subtropical India.

Yep I request ideas about acacias!

Forgot to tell about prickly pears: yes I have some that are more spiny,
but 2 drawbacks:
- I leave some I already have, but cannot let them near the orchards, as their roots cross the distance to my avocado watering!
They compete well...
- They grow slowly to the size they become woody and strong. Young pads are easy to break.

We also have yucca sp. but which species? I made y. baccata from seeds. also y. arkensis known as indian cabbage, but none went out.
Did not notice so many spines in yuccas, and they grow into a smooth trunk.
So I did not hink about them for a fence.

Blackberry plant with fruit in my garden & Yucca fence, those leaf tips are NASTY

I am going to change the place of my recently planted yucca... A place that will honour the nasty pointes leaves then!
There is obviously the same yucca in Tenerife and here in La Palma, just in case you know the latin name, as there are many yuccas...

Here are some pics of dovyalis, from my neighbours, you can see the size of 1 plant. 2meters. It is 8 years old.
And nicely armed...

3rd pic, this is what I can consider an opuntia fence...
It is at the bottom, rooted below the garden, so I guess it does not suck too much.
You cannot really see that there is a ravine.

By the way you can also see the preperation of part of a sort of hugel surrounded with stones in the corner below, left.
We filled it with some of those pads.

Fennels are bigger than my carissa plants, I still have to mark them with a stick.
But they already bear their 1st flowers!

Ok, I looked at osage orange, and it looks good, apart from a doubt I have to get it to grow with no extra watering.
It seems to like some water or grow near rivers.

The acacia polyacantha is south african but is said to be more for having a tall tree, and is from a wet subtropical place.
"The tree often grows in the moist, subtropical bushveld of Africa, usually in alluvial soils near rivers."
http://www.plantzafrica.com/plantab/acaciapoly.htm

Actually, I did look at all the yucca around, and none has nasty spiny leaves!!!

And I have found 2 new volunteers....

1) It seems that some pomegranate are spiny, probably old non comercial varieties.
I will get some seeds from fruits around and try to grow them.

2) I also have a fast growing idea, a bush:
tajinaste.
A local plant growing over 1 m can reach nearly 2 meters, very good for bees and drought resistant.
Someone can easily cut it though the base will become woody, but you must really want to go through it....
I already have some, just have to direct them to where I need them...

It is actually endemic to La Palma!
https://es.wikipedia.org/wiki/Echium_brevirame

Tenerife has its own:
https://es.wikipedia.org/wiki/Echium_aculeatum