Bots

[Silverthorn]

Or. Protect the forums with password, and only guys who PMs admin or mod gets a password and can come in.
This should be announced in easy way - so noobs could understand.

...let me elaborate this a little further.

One day, a guy joins, asks staff for password. Everything is alright so far. Too bad that this guy sets up bots and feeds them with the respective password. Bot-invasion still there.

tl;dr: ineffective.

Also, while we're talking about bot-invasions, after the last rage-ban-spree, there hasn't joined any new bot. jfyi.