They are great for your tank's ecosystem...it's really much better to just leave them be. Keep in mind that other people would want that rock just for what you say it has on it, so if you hate them that much, you could probably put the rock in a bucket to keep it wet and either sell it or trade it for another with fewer critters. If you try to pick the stars off you'll probably just make them drop their arms and mangle them. If they are on one rock right now and you can be patient, they will eventually disperse to other parts of the rockwork and generally make an effort NOT to be seen, since even larger brittlestars prefer to hide.