Laki, you can use the freezing method but there are some who say it's inhumane and some who say it's not. Since I'm not sure myself, I don't recommend it just in case it really IS inhumane. There are some websites that do have good freezing methods though, preferable to just sticking it in the freezer.
Bettas, clove oil acts as an anaesthetic and when overdosed, it basically puts the fish to sleep. But it's a hard method to use because oil and water don't mix well and if it doesn't work right, the fish just ends up getting agitated. I don't use it any more as a method.