We use 4.5mm diameter pieces of very fast moving lead
.
Seriously though, after our rats became trap-shy, we got rid of the rest by baiting with Raco poison blocks. Prior to that, we tried
"Eradibait", which is marketed as being non-poisonous to hens and pets. It looks a bit like the wood pellets you'd use in a boiler, and is meant to cause death by dehydration.
I'm sure it works..... if you can get the rats to eat it! We tried all sorts of things, including mixing it up with chocolate and peanut butter, and leaving bags of it by the rat runs. This was a total waste of time - the rats just licked off the good stuff, and left the bait! An utterly pointless waste of rather a lot of cash - you have been warned!
P.S. If anybody's desperate to try it, let me know. I have about 30 quid's worth left that you can have if you'll pay the postage!!