Is it all on one rock? Easiest way IMO is going to be removing the rock and letting it completely dry out. Aside from that, there are a bunch of remedies with mixed results:
Kalk Paste right on the colony
Reef putty right on the colony
Injecting with various things ranging from kalk (base) to vinegar (acid)
Injecting with "over the counter remedies" such as aiptasia X, sold at the LFS
You can also try peppermint shrimp. Those are hit or miss. I have actually seen a picture of a (large) aiptasia eating a peppermint.
There is also the berghia nudibranch, which I have heard people having great success. One caveat is that you will need a varying number of these guys to actually mow down all aiptasia - dependent on how many aiptasia you have. They are also quite expensive (about $15+ a pop), but can reproduce pretty readily if given ample food. They ONLY eat aiptasia, so they will starve once they get rid of them all. Upon death, they release a toxin into your tank that can wipe everything out pretty fast.
Lastly, there are some fish that have been known to consume it as well, though none that would do well in that size of tank.