I suspect if a member was banned from a site running forum software - be it SMF or Vbulletin I suspect when the account was accessed a message would deny login and state 'you have been banned' or email admin re your account being locked.
Not always the case Marty. There is a mod for SMF (which we can't install) as well as vB called "Miserable Users". Basically it causes a pre-defined list of users to get server time-outs. That's possibly what happened here.
He he he...I can't wait to get vB! 
Just for info...
vBulletin 4.0 VersionPlease remember to click Mark as Installed if you use this modification.
Please fee free to nominate this modification for the MOTM, and/or use the donate link provided (Support Developer).
The ever popular "Miserable Users" modification.
Basically a way to really annoy anyone you don't want visiting your forum. Credit to Zzed for the original idea.
When classed as miserable, a member suffers ;
1. Slow response (time delay) on every page (20 to 60 seconds default).
2. A chance they will get the "server busy" message (50% by default).
3. A chance they will get redirected to another preset page (25% & homepage by default).
4. A chance they will simply get a blank page (25% by default).
If they get past all this okay, then they will be served up their proper page.
Note: All the above are set in vBulletin Options > Miserable Users.
