with the above minus the killing part. Killing is a hard thing to do well. Of course if it goes on for weeks or months then it might be the only option.meh this is an easy situation
1- make sure it is what you say it is
2- find the reciept for the fish
3- go to said LFS and tell them you want your money back. Don't ask tell. If he gets lippy, drop the bomb that its an illegal fish and if he doesn't give him your money back, the police/whoever deals with this will be back with him tomorrow
4- profit
if you don't want to do that... kill the fish. people who house illegal fish just bring problems to all other fish keepers. Be responsible... suck it up and do it.
I have herd that freezing is actually very painful for the fish as ice crystals can form on the skin minuets before its actually dead. Unless you held the fish underwater some how so its skin would never be exposed to the air.