For those of you who've imported from another product and want to purge welcome messages, heres a quick addon for you.
Simply drop this file on your webserver and update your sql data, and edit the $welcome section, load the script in your browser, and any messages matching $welcome% will be purged.
$welcome is the string you want to match, and the script automatically links "like" topics (such as welcome to our forums, welcome to our site, etc. with the default match of "welcome".
This addon will purge from all pm tables, and not just the topics and posts table like prior mysql matches did, this will provide a CLEAN purge of welcome messages, versus only purging data from 2 of the 3 tables.
I will be working on a way to repair the users table for "botched" welcome message purges using the "alternate" means of just deleteing items from the database, not sure on an eta.
This should work anywhere UBB7 would work. If you have problems, let me know!