Honestly a motivated pet - cat or dog is the best answer. I got our labs to rat which is awesome, until one - bless her heart - ran flat out smack into the barn wall and knocked herself out. Now I rely on our cat, who is crazy about bringing us live-ish rodents then consuming them in front of us, ( whole may I add) with such happiness I don’t have the heart to vomit anymore I just cringe and say “good Kitty”. If you find a network of tunnels you can duct tape a heat resistant tube (Dryer duct) to the tailpipe of your car and put it over the hole so you CO2 them to death. A small engine like a tractor works well too, just takes longer. Some people swear by those small high frequency spikes that run off of solar or batteries with the high pitch frequency. I personally have found them helpful but not enough to say they take care of the problem. I have had to use a multiple pronged approach with the noise pest control (some of the brands work - others like from home Depot don’t at all), control through cats/dogs/wildlife, and manual traps that you put out but won’t harm the “hunters” of the voles/rats/mice. I have also heard peppermint or essential oil deterrent helps but won’t fix it by itself. Just my two cents, good luck and I know it’s hard but poisoning them, I’ve seen that go wrong too many times and like other people mentioned - it does go down the food chain pretty far. The rats, if I poisoned them would have killed the red tailed Hawks that prey on them - that was great 3’ away boom! Downed rat and the hawk got such a fat one he had trouble taking back off! To the foxes and onwards up, even neighbors pets. Good luck.