need help stopping signups to spam: "send email to user" conditional upon usergroup

[boatdesign]

How do I stop someone from signing up to spam email my members via the "send email to username" option?

I elminated PM spam by new signups by making PM system access a usergroup option so you need x-number of posts to send a PM (usergroup promotion automatic by vbulletin.)

But I can't find the option under usergroup settings for the send email to user option? I've checked the box to make it secure (meaning via form vs. simply displaying the email address) Can a plugin or other mod be devised to make "send email to user" form conditional on usergroup? Otherwise, someone can simply signup, and with 0 posts, start sending spam email via vbulletin to other vbulletin members which is not good