Most folks use Joes Juice or Aiptasia X to get rid of them. I've had good success using lemon juice injected into the body of them using a needle and syringe. If you can get the rock out, just chip them off using a screwdriver. You can also cover them up using two part reef putty, this works well if you can't possibly get the rock out, or get at them with Joes Juice. I did this to a big aiptasia that was hiding in a crack, he never showed up again. Some folks will take the rock out and burn them with a lighter or soldering iron.

Ball nems are not as hard to get rid of as aiptasia, IMHO. Usually one treatment does a number on them. Aiptasia, otoh, sometimes require the use of high explosives.