It depends on what the member is doing. VPN or not, when a member is doing strange things or acting stupid on my forum, then there is need to do something as quickly as possible on it. VPN seems not to be the issue on forums, but what those behind them are doing on such forums.
Personally, I don't think it is necessary for the use of VPN to be banned on forums because it is something people make use for security reasons. The only way I would support banning of an account user who is making use of VPN is when the person is creating multiple accounts on the forum.
My own general policy has always been that, if someone wants to be on a site I may own or run, I don't want to restrict their access unless they are doing something on the website itself that warrants a look into IP history to determine if an IP block needs to be done. And it is easy enough to block ASN's to make sure certain VPN services aren't usable to gain access, like blocking a VPN service where spam generally ends up coming from.
