IP banning has lost a lot of its effectiveness when ISPs started putting people behind NATs and recycling their IPs (which was a long time ago), and further lost effectiveness with the advent of smartphones and mobile Internet connections.
Ip banning for new accounts together with creation time exceptions seems pretty good thought: almost all existing users would be unaffected. Only costs you growth untill you find some extras to put into the account creation process.
I mean you could try to keep up with Tor exit nodes and VPN gateways but pretty soon you'd be banning all of Azerbaijan or something.