The humane and only propper way to do it is CO2. Check out the feeder section. You can make a CO2 euthenizer to use long term for pretty cheap. If you don't have the cash, you can just use a cheap tuperware container and canned air for now.

Many people knock out or break the back of rodents using various methods. This is NOT an acceptable (or legal in most places) way of doing things. It is cruel, inefective, and messy.

If you can't do the CO2 right now for whatever reason, just feed live. In my opinon there are only two acceptable ways to kill a rodent feeder:

1.) Co2 eutheniztion
2.) Snake constriction