These little monsters are evil plain and simple.  There are many ways to deal with them that are often found on the net.  What they often do not tell you is how not to deal with them.  Here is where I would like to add my two cents.

    First off never feed them, they can act like tribbles, or gremlins, if you do.  Next up is never, never try to mash, poke, cut, or otherwise damage them without completely killing them.  If you do they will multiply like rabbits, or faster.  Do not play with them by making them retract rapidly.  Yes, I know they can move fast, but this seems to be a trigger causing them to again multiply.  When you are ready to kill them, know that if they retract into the rock and any part of that survives, it will multiply.

    Here would be a good place to point out that even I must have let one slip into my show tank.  I killed it, then 3 months latter noted another only about 1/32 of an inch or so tall.  I killed that one surely, and haven't noticed any others.  I'm sure I will see another or more latter, they are very resilient creatures.  I think that if you catch them when they are small you have a much better chance at killing them completely.

    One method I use to kill them is to make kalk paste at about the thickness of room temperature cake frosting.  I then take that and apply it to the rock around them first, being careful not to disturb them more than necessary.  I even try to get it into the rock if and where possible.  Then gently cause them to retract, cover them up completely with the paste then wait a minute.  Then poke them in the center while injecting even more of the paste.  The idea here is to encapsulate them in a tomb of sorts, so even if a couple of cells live they have nowhere to go but into the paste which then kills them.

    Another method that has worked for me if the rock they are on is small is to take the rock out and dunk it for 10-15 seconds into scalding hot heavy kalk water.  Then rinsing the rock in a container of tank water that I then dump down the drain.  This works well with small rocks, but I would never attempt it on large ones.  I also only try to dip as much of the rock as absolutely needed as it will kill virtually all life that is submerged.

    Having taken care of most of mine in my seed tanks prior to allowing the seeds to be placed in my show tank helped a lot.  While doing this I noticed several traits that may be helpful to you.  They do not seem to like direct sunlight, low PH water, nor high water flow directly on themselves.  They also apparently hate their own if they have any color differences.  Light and darker ones tend to move away from each other.  Although this information should be taken lightly, it is what I have observed.