We have an alum who is playing around - setting his name to nonsense, having garbage in his street address. I presume I can prevent him fro logging in by changing or deleting his primary email address. How can I also for him to logoff. I hope there isn't much other than sending messages I suppose that he can do to wreak havoc. I need to check further into this alum. Thanks

Here's one thing to consider:

1) Delete his account entirely. That will immediately stop him from being on your site even if he's currently logged into it.
2) Click your Preferences link and turn on the New Classmate Verification feature. Hopefully you won't have to leave it on too long, just long enough until this guy goes away.
3) Now add his name back to the Class List (if you want to)
4) If he attempts to join the site again just don't approve his account. He'll be able to log in but nobody will ever see him and he won't be able to post anything anywhere, etc. He'll be rendered completely powerless.

Note: If you do this the Classmate might get frustrated and attempt to join using names of other unjoined Classmates. If this happens simply delete the account he hijacked and just add the name again to your Class List, allowing the real Classmate to use it. It won't be long before he grows tired of this game.