How do I limit this to one blog per account

I have a multi-site install with pro member plug in how do I limit users to 1 blog. On the admin panel when you click on get another site, it says
Get another site in seconds

Welcome back, Username. By filling out the form below, you can add another site to your account. There is no limit to the number of sites you can have, so create to your heart’s content, but write responsibly!

Sites you are already a member of:

If you’re not going to use a great site domain, leave it for a new user. Now have at it!

HOW DO I limit this to one blog per account?