I use metal tweezers and tear them off from the base. Once you do it enough, you'll see that there is a grain or attachment direction and you can start the tear from the weak side to get the most tissue removed from the rock. Just put the excess down in the fuge or give it away for free or a frag (there are always takers).
It usually takes a little while for the leftover tissue to grow back again and if you continue to keep it trimmed it won't have a chance to spread anymore. Other than that I guess you could use a tank friendly brush to scrub the rock and get more tissue removed...
Zoos & GSPs tend to defeat anthelia/xenia so if you don't mind them, they'll usually take over their place.