a few questions

1) Usergroup settings for whatever usergroup you'd like to increase PM storage on.

AdminCP -> Usergroups -> Usergroup Manager -> (pick a usergroup) -> Private Message Permissions -> Maximum Stored Messages.

2) Ban them by IP address. You can see their IP by viewing their user profile in the control panel. Make sure to ban the EXACT IP, not a partial, otherwise you will risk inadvertently banning other users.

AdminCP -> vBulletin Options - > User Banning Options -> Banned IP Addresses.

Additionally, you might want to check out the Miserable Users Hack, as well as the Multi Login Detector.

3) Looks like someone's having fun with your home PC, unless you're using the same exact login everywhere. The first thing you should do is change all of your passwords, and make them unique so that whoever edited your post here can't do it again.

As far as the couple of hoops to jump through to be able to post here, you'll come to find them well worth it. The coders here put a LOT of time and effort into the development of the hacks/mods here. One thing that it does is protect them (as much as can be) from pirated vBulletin users downloading and using their hard work on boards that they didn't buy a license for.