Make sure it doesn't contain bitrex in itGas them with the fumes from an engine, i know you said you cant use poison, but vermin seem to like the sweet taste of antifreeze
I never knew they put Bitrex in antifreeze. I used to put it down for the mice to drink in an old house i used to live in. I cant imagine anything being resistant to antifreeze poisoning.Make sure it doesn't contain bitrex in it