Simple Machines Forum spambot zapper

[#1]

I thought to myself which is the best forum software? I then thought, which one does plusnet use, they should know which is the best so I installed SMF on my plesk server space for a rugby orientated discussion forum.
However, my most frequent visitors are spambots creating fake memberships to advertise (some less than desirable) I use the strongest captcha mode on the standard install and still no joy.
I was wondering what extra stuff does PN use to block these attackes, only the mods I've tried won't install.
The site is currently switched off until I get a solution 

[#2]

Have you tried http://custom.simplemachines.org/mods/index.php?mod=1547
I'm going to be converting my phpbb forums to SMF once 2.0 goes gold and I'm looking at this mod.

[#3]

Quote from: Word

I was wondering what extra stuff does PN use to block these attackes, only the mods I've tried won't install.

They ‘use’ the fact that only Plusnet customers can register to use the forums.