It may also help to maybe not except VPNs during registration. While many use VPNs nowadays, it isn't needed for a HTTPS registration here, and also would prevent a majority of modern bots piggy-backing on free VPN services posing as legitimate locations as well ISPs/Hardware.
SMF is really really outdated these days, more maintained than active development. It's a good base for heavily customization but as is, without good plugins or custom hooks, it's pretty plagued with issues.
Additionally maybe just adding the simpler user group privileges scheme might do wonders. Ask users to introduce themselves in a specific forum before moving them to "Members", where they are allowed to create threads and replies with the rest of the forum. Effort on the administration side is drastically reduced, easier to cleanup, and doesn't effect where active members are interacting.