Being a member and former president of the Gulf Coast Turtle & Tortoise Society, may I add my 2¢ ?
I don't know how to discourage them, other than a large dog. And who wants a mauled turtle?
You could try fencing them out, but they dig, AND they climb. Trust me.
I think driving them elsewhere is about the best idea. BUT... be aware that the same "good" bacteria that live in our fish also live in other animals. The GCTTS is a big advocator of NOT displacing turtles to an area with which they are not acclimated. Because they could introduce some new "bug" to the local population and / or fall prey to something the locals have resistance to but the new guy does not. Whole populations of our Texas Tortoise have been wiped out in the wild by people thinking they were doing good by letting a "pet" go, but in actuality, it introduced a sort of plague that killed off the native population.
So... if you do drive them somewhere, be sure it's not too far from where they are living now.
Or, alternately, hows about giving them their own buffet... set out food expressly for them? And include some hiding places nearby, so they feel secure, and actually eat what you want them to eat, not your garden fare.
