Here's what I do to get rid of them, and it seems to work great:
Get a shot of tank water, add in a tsp or 2 or salt to make it 'heavy' so it sinks.
Mix in kalkwasser paste until it is the consistency of mayonaisse.
Shut off power heads and any flow that is easily shut down (i leave my return pump on and it doesn't cause problems)
Using a 5 ml syringe, put the paste onto the aptasia. Cake it all over it. The aptasia will probably try to eat some of the paste, and that is fine.
Wait 10 minutes, then suck the paste and aptasia out using a syphon hose. Once the paste/aptasia are removed, turn the flow back on in the tank.
And its gone! If you miss sucking it out, it will probably open back up in a few days, and you can try again.
I've tried many methods, such as injecting them with various things, but this seems the easiest (since you don't have to inject the stalk) and it does a great job of getting rid of them without them spreading.
rob