I had a VPN buster bot on an IRC channel I was moderating because of some persistent troll who kept switching between every VPN that is out there.

Apart from using a deny list that you can find on Github, you just look at the size of the subnet the IP is coming from in WHOIS output. VPN IP adresses are part of very small subnets unlike legit ISP ip addresses that come from wide subnets

https://github.com/ugjka/flipbot/blob/master/trigger_vpn.go (it is not pretty, i know, but it worked)