I have used the kalkwasser method before. It works very well. I actually used pickling lime (same stuff). You can pick up a giant can of Mrs Wage's Pickling Lime in the canning section of most Wal Marts for just a couple bucks. Mix with water for a thick paste and squirt on to the aptasia. Make sure to pile it on, covering the thing. The paste will disolve over the next week.

ANother option is to pull the infected rock out and go to work with an ice pick/screw driver. Make sure to complete blast the area leaving a sizable crater. Rince and return the rock to the tank.