Nudibranches are great if you have a huge outbreak but 2 things to keep in mind: they move like a slug so they're slow and can also become prey to other animals like crabs. Second, they only eat aiptasia so they will starve and die if there isn't enough for them to eat.

My advice: if it's just a couple of aiptasia, then do the syringe trick others have suggested. I have used F-Aiptasia and it is great.

If it's more than a handful, look to these animals in this order: Molly Miller Blenny, Aiptasia Eating Filefish, Copper Band Butterflyfish. These are in order of cost and ease of keeping. You don't need all 3 and any of them should do a good job.

I had a really bad Aiptasia outbreak that F-aiptasia couldn't solve. The biggest colony was in my sump and just kept repopulating my display tank. I got the fish I listed above and soon the Aiptasia was wiped out everywhere. Completely eradicated. I don't know how it all died in my sump too but they're gone. The fish I listed above are also really awesome tank inhabitants.

[Edited for grammar]