If the trees aren't too big you can put a net over them.How do you keep squirrels from eating the fruit on a fruit tree?
aluminum foil scares them almost as bad as my dog
Hang an plastic or wooden owl in the tree. The squirrels won't go near the owl.
The baffle idea is good. I use 6'; stovepipe on my bird feeder. Very inexpensive.
I finally gave up growing nectarines. Every year I got fewer until I got none. Moved. Now I have the same problem with grapes and plums. Mesh doesn't work unless it is hardware cloth.
Some types of critter urine are said to repel them.
The Plastic owls only works for about a week, but if you time it right when the plums ripen, that may keep the away long enough for them to ripen.
If there aren't other trees around for them to jump from, one of the best things is a baffle that goes around the trunk of the tree. Its a plastic cone that goes about 5 feet up the tree that squirrels can't get around. Its pretty fun watching them try though. Of course if there is another tree from them to jump from, that wont work very well unless you baffle all the trees.
You can buy baffles at your local large hardware store (Home Depot or Lowes) or at Wild Bird Centers.
cut the tree down.....