I purchased a home last year, here near Fayetteville, NC, and the backyard was slam full of sandspurs and every other weed known to man (it was a foreclosure).
I've spent the past year renovating the back and front lawn (as well as the inside of the house). I thought about every conceivable way to get rid of those sandspurs and almost went as far as plowing the whole backyard with a tracter or something and starting from scratch. However, I was also afraid that the sandspurs would just get mixed around and end up sprouting again once I've planted new grass.
Then I was discussing this with my father in-law who owns a landscaping business mowing lawns and stuff. He suggested I use his johndeer mower with this vacuum attachment that is on it that has a separate motor that vacuums up anything you mow into this big bagger that is pulled behind the mower. So I went and got it from him and used it on the backyard. When I did it, I put the mower blades all the way to the ground as low as they would go. I even took the wheels off the deck so it would lay on the ground. The blades were actually mowing the dirt and everything got sucked up into the bagger. This is a pretty big back yard so I had to empty the bagger thing numerous times.
I did this in April and then put down my grass seed and proceeded to water
it everyday for about 3 weeks. After I mowed it and sucked everything up, I couldn't really find hardly any sand spurs out there. Before I could take 3 steps and my flipflops would have tons of them stuck on them.
Since then, my grass came back great and I didn't have any spurs until about a month ago for some reason some started coming back (I guess august is when they start sprouting?). So first I sprayed the back lawn with Image herbicide. It says it kills sandspurs and it's safe for centipede grass. After that, since there weren't too many sprouting, I decided the best thing to do with these was pull them up using a weed popper. The entire plant and roots
comes up pretty easily. After I pulled what I could see, I then mowed my lawn at about 1 inch and used my regular bagger attachment to collect all the clippings to catch any spurs that I missed. I figured any I didn't pull would die from the spray
and I would just pull any others that I see from here on out. It's been about 3 weeks and I haven't seen any new ones sprouting.
Hope this helps. These things are a beast to get rid of.
If you're talking about a very large area, you're only option may be to just spray
it all with the image stuff, and maybe do it a couple of times a season for a year or two and you'll probably eventually get them all. The problem is, you can only kill the plants with the spray, but the spurs are still there and will probably sprout again the next season, so you really need to try and get them up as well.