First of all, I would like to apologize to you for taking so long to reply to you ..
To be honest, I totally forgot about your problem .!
Anyway, I think that I have spotted the problem and I believe that it is in your vBulletin options and not in your design ..
All I want you to do now is to follow these steps:
First of all, sign into your Admin CP ..
Then, go to "Setting >> Options" ..
After that, choose "User Registration Options" option from the settings type ..
Once you enter the options page, look for "Allow Multiple Registrations Per User" ..
Make sure that it is set to "No" and if it wasn't then set it to "No" ..
Don't forget to "Save" your settings ..
What I actually did to spot your problem was to set the "Allow Multiple Registrations Per User" to "Yes" and then check my test forum..
This idea came to me when I clicked the "Register" button in your forums and as a registered member I shouldn't be able to enter the register page.!
By the way, if this solution didn't work for you, we're gonna need to play with the conditional sentences in your "header" template but I don't think that we'll need to do that ..