If it's a big rock you would rather not take out of the fuge I've found that simply encasing them in superglue kills them and prevents them from ever spreading... kalk paste works good too. But you could also let the rock dry out if it's small... however I must say I like your pyrotechnic idea of making an Aiptasia grilled well done. And I've never thought of what twitch said of asking the lfs if you can put it in their peppermint tank, but that's a good idea.
I should write a childrens book about this.