I couldn't find anything on their website for delisting, but it might be something your ISP will have to do if PerimeterX has a delisting procedure.
The other possibility is your ISP has let the VPS traffic go over the broadband ASN (either accidentally during a failover event or on purpose to help with load) and if that's the case it might just have an IP reputation too poor to be useable.
Anyway you can use a VPN?