Admin CP >> Display Options >> Languages || Language Editor

Either...

choose - Choose Specific Language File: adduser.php and select WEL_MSG

or search string 'Welcome' = and once again edit adduser.php

Quote
Welcome to our forums! Please take a moment to review and update your profile and preferences to take full advantage of our features. You can do this by clicking on "Edit Profile" and "Edit Preferences" in the My Stuff dropdown.<br /><br />Please do not reply to this message as this is just an automated welcome message to thank you for joining our community.

This may be a slow way of getting there - but it will get you there wink