Assuming you can readily see these guys, I'd get a break barrel pellet rifle. It's plenty accurate and powerful enough for the job, safe, and won't make a lot of noise.
For instance:
http://www.pyramydair.com/cgi-bin/model.pl?model_id=297
http://www.pyramydair.com/cgi-bin/model.pl?model_id=551
Both about $85, and would be suitable for precision vole sniper work.
I am no expert on the poisons, but it seems that if you used small amounts of a biodegradeable poison, and placed it right into their burrows to minimize the amount used while maximize the likelihood of their taking it (also not someone else)... there wouldn't be too much risk with that.