Hi,
Look up
Permaculture HA-HA fences. Basically a 2-3 meter wide ditch with very steep sides, elephants stay on one side and your farm or garden goes on the other.
Also do not over look the use of electric fencing, it has been effective in reducing human-elephant conflicts. The single largest problem you will have is the incredible sense of smell that an elephant has. Simply hiding desirable food items from view is not
enough, you must also work to confuse their sense of smell and obscure the scent of the more desirable foods as well. Finally since they are already visiting you could also try to plant some of their favorites in other locations away from your farm (perhaps in the forest they come from) to help divert them away from your crops. I would imagine the
Sepp Holzer "bone sauce" might be quite effective.
For best results I would HA-HA or electric fence my farm or garden area, both if possible. I would then plant lots of smelly plants out at the edges as well as interspersed throughout the garden. Beyond this I would work on creating the bone sauce and bone sauce everything I found reasonable at the borderline. I would then also work to plant some desirable food items for the elephants in a different location away from the farm or garden in a place of my choosing.
Good Luck,
Jeff