One of our clients recently installed vBulletin and has seen a fair amount of spammer registration on their site even with image verification, email confirmation, and new user moderation. In the last couple of days we have noticed newly registered users, but not approved, send private messages to spam other members of the site. In investigating how to prevent this I learned something new that I think is not made as obvious as it should be by vBulletin.
To insure that private messaging disabled for a usergroup (ie - users awaiting confirmation email or awaiting moderation) there isn't a master switch turning off the feature for the usergroup. You have to change the "Maximum Store Messages" to 0 for the usergroup when editing the Private Message Permissions for a usergroup.
In this section set "Maximum Stored Messages" to 0
Frankly, as an administrator, I would like to see
Jelsoft change it so that there is a master "Yes/No" setting for whether or not private messaging is enabled for the group. Unless you look closely Maximum Stored Messages doesn't look like that switch even though it is used that way.
Keep you spam and moderation to a minimum by checking your usergroup settings for the proper maximum store messages setting.