Simple fix, Go to the FORUMHOME template and find either
Code:
<div><phrase 1="member.php?$session[sessionurl]u=$newuserid" 2="$newusername">$vbphrase[welcome_to_our_newest_member_x]</phrase></div>
Or if you can't find the first code, find this
Code:
<phrase 1="member.php?$session[sessionurl]u=$newuserid" 2="$newusername">$vbphrase[welcome_to_our_newest_member_x]</phrase>
And remove it.