I temporarily created my own forum so I could look around the Admin Panel, and there may be a solution...
General Admin -> Configuration
Set "Enable Account Activation" to Admin.
Then, you could implement Rock Quarry's idea as a confirmation email of some sort.
OR, if you can, switch to IPB. There's a feature in there that allows you to moderate the first X posts of a user, as I belong to an Invision board that has that feature turned on.