Man, if only we didn't live in the frickin' suburbs, I'd be saving up for a BB gun right now.
Anyway, I think I finally figured out a way to keep them out... my outdoor plants (see growlist, everything except Nep, Ping and adelae) are all on the top shelf of my greenhouse (looks exactly like the one behind NickHubbel's head in his pic in his profile, except mine's orange), and I recently put the vinyl covering back on it, cut out two large square panels from the top so the plants could get direct, unfiltered sun without frying, and then, since both zippers on the front flap of the covering broke (@#$%...), I took a bunch of safety pins and pinned it shut, spacing the pins about 4 inches apart. The squirrels got into the greenhouse through the top, which I expected, so I took one of the lower metal mesh shelves that I had no use for at that point and time and placed it on the top two parallel bars above the top shelf, so now the squirrels can't get in unless they figure out for to lift the shelf, since one top plastic part of the greenhouse was assembled BACKWARDS, so due to the arrangement of the notches, I couldn't actually snap the two bars into place to set the shelf on top of, so they're just sitting up there. Anyway, hopefully the squirrels won't figure out how to get in through the spaces between the safety pins on the front flap, but they haven't so far, so we're good.