I'm irritated by this thievery even though they are not my trees.
I would suggest coating the trees with some noxious, but non-toxic substance; animal manure maybe or a non-toxic dye/coating (latter in a non-Christmassy colour - e.g. black) before the "season". If contamination of berry clusters is avoided, the birds etc won't be put off I would think. (The trees will almost certainly benefit from a leaf-coating of animal manure/slurry.)
Even forcefully lobbing a few whole cow-pats, a collection of sheep droppings or horse droppings into the trees might be sufficient deterrent aided by signage .. "These trees have been doused with ****** (e.g. cow s**t). We wish you and your customers a smelly Christmas."