Freezing isn't the most humane method but certainly the easiest. If you have access to a -80 freezer or anything like that, use it. There are a couple of other options though. Clove oil works well. I think I've seen it at Walgreen's or larger chain grocery stores.
Ideally you'll want to find some MS-222; unfortunately it's tough to find. If you know anyone in an ich lab, they can probably hook you up. Just be careful with it, use gloves and don't sniff it.
Now that I typed that up I remembered Cornell and the AVMA published guidelines:
http://www.research.cornell.edu/care...Ps/CARE306.pdf
If I remember correctly, you can get the benzocaine hydrochloride at the drugstore in the form of cold sore lip treatment. If you're only knocking out killies I imagine it wont take much of a tube, depending on concentration. It might look strange anyway if you were buying 10 tubes of cold sore treatment =P