Try to be a bit more open minded and accept people's suggestions and input. If you are asking as a regular vbulletin user, learn from what the more advanced users tell you. Being set in your own ways and thinking that you know the problem will make it so that people can't help you. If you know it all, then you shouldn't be having this problem
It
can be a server issue because user's
can give do whatever they'd like with any account (including there's) via phpmyadmin and such. Also, if it keeps happening to the same user, that is rather suspicious, is this the case? If so, it can be a problem with the user. Try giving every usergroup regular user permissions accept for you (temporarily), and see how things go.