Taking a trick used by Steve Irwin, the Crocodile Hunter, to pursuade a
large croc to stay away from human habitations, how about this:

Catch the squirrels in a live trap.
Place the trap with squirrel near the spot where the squirrel did the
damage.
Spray the squirrel with a water hose several times, or do some other
humiliating thing to him.
Let him go.

The idea is that the squirrel will associate a distasteful experience with
the plants he's been destroying and not come back.

I've thought about using this to "train" racoons, but have never tried it.