Remove "just a user name please" from pro-site/multi-site signup page

I am new to setting up pro-site/multi-site signup settings.

I have an option (at the botton of the signup.php page) which gives a choice for:-
1) "Gimme a site!)"
2) "Just a user name please".

How can I remove option 2) please?

I wish visitors to only be able to register a site for active use!

Thanks in advance.