a benjamin sherridan air rifle will kill them. benjamins are pump rifles which cost about $120 and come in .22 or .177 (commonly)
they have a lot more mezzle energy than any cheap
spring-over-air air rifle.
they are a hell of a good field target gun if you don't mind pumping.
if you do mind pumping they can be converted to run on co2 or hpa. here's mine which I converted. (I have a small machine shop)
it's the funny looking scoped rifle on the green pool table.
http://www.spaceman613.net/machining/machining.htm
then again that ar15 would be a better choice.
but really, if you don't have the tools to go through all that trouble of fixing up a shen or anything extravagent a bungee over the lid will probably do just fine.