I think some forum sections should be "private", and guests can't see what is happening there. They can see them after registered.
- News and Announcements (If guest would be interested to see what updates there are, guest is ready to support and register to forums)
- Events & Competitions (Same as above, guest support server by registering to forums)
- Bugs (Guests could use exploits, if they find ones from the bugs section)
And also forum need some other upgrades like these.
- New users can create new topic after 5 or 10 posts, or account been active for week or two.
- Timelimit for new created topics, to prevent forum alts spam on the forums.
As above suggestion, I think there should be actually reason why would people register on the forums. To support server.