I have a 'Validated Users' group with a usergroup promotion from Registered Users based on Trophy Points. The Validated Users group then has greater permissions (eg. edit sig, post profile posts, upload attachments, and content creation via add-ons (eg. wiki, events, directory, etc.)). Essentially, my Registered Users can only post messages/topics in the forums until they achieve the trophy points for automated promotion to the Validated Users giving them much greater content creation permissions.
It appears to work well to keep out many of the human spammers using sigs and profiles, without being overly onerous on proper users. What measures do you use?