Freezing a live mammal is incredibly cruel, ice crystals form in the blood and it is a slow painful way to die. Drowning is horrible as well, I can't believe that is being recommended.
Humane euthanasia is the way to go, one should show value for all life. C02 is the preferred method, if you know how, cervical dislocation can be done but if you don't know how to do it, then you may cause un-needed pain and suffering that way as well. I've also thumped them before (blunt force head trama) but I don't deal with live feeders any longer.
OP if you snake won't feed, can you take it back to the store then? Don't just toss it in the freezer, please.