Pretty common issue most anywhere.
If your able, hang them out of reach from the ground. We put ours about 10 feet off the ground. If you can't hang them out of reach, might use metal pipe (capped to close the open end for the critters) with the signs bolted on, concreted in the ground.
I just had to replace the location marker on one of the GPAA claims our Chapter adopted. Someone cut the cable and broke open the tube.
On my own claim had to replace the location notice that someone urinated on and put back in the tube. (laminated the replacement to solve that)