how do YOU combat spam?

I turned off guest posting and that did it for my small boards

one way is to alter the way registration works.  add a users_temp table that stores the users record w/ the addition of 2 extra fields which are 1) a ramdomly generated digest (not a hash of the password), and 2) a plain text associative word.  when the user signs up, they are displayed an image of an object (which they're told to remember.  this is a simple one word object like circle, or square, or something very very simple).  when the user submits their registration the link in the email has the random digest token, and takes them to a page where they then have to enter in the word to match the picture they saw when the signed up.  when they've successfully enterd the right word they're added to the users table (and removed from the temp table).  if they miss the word 3 times, their record is removed and they're told to re-register.

this will cut down on the number of people who provide bogus email addresses, but know the formula for generating the hash of their password, and posting the authorization post that finalizes the sign up process.  once you keep them from registering, then the random spam should also stop.

Bekko: Or allow it to be on a per-usergroup basis and someone can write a mod to automatically graduate a person when they reach x number of approved posts

Yeah, but more integrated and automated within the overall PunBB system

For me -I want a forum opened to guests- askimet plugin made for punBB is a working solution. Some errors but it's ok at this time, although I think it may be better.
You may also have a look at Tobi's code, counting numer of url in post : http://punbb.org/forums/viewtopic.php?pid=76114#p76114 or Elbekko's mod : http://punbb.org/forums/viewtopic.php?pid=76146#p76146