I was having trouble with signs disappearing in some areas just as fast as I put them there. It was getting kind of tedious and expensive, so I became pro-active and problem solving.
Super Signs were the result. Using metal from a bed frame and plywood, a battle ready sign was made and a normal sign fastened to it. Then its sharpened end was pounded into the ground with an old fashioned Post Maul.

It was placed in the worst area for sign destruction. After several days it was still there, but one night the plywood was torn off of the pole and taken away. This showed the weak spots, and also showed it wasn't casual damage. It took a determined effort to rip it off.
Using the info gathered from the beta, a dozen more were made with improved design and construction. So far so good.
The only problem is I have no idea how to get them out.