Registration of Subsites on subsites

Hello,

I am wondering is there anyway to "disable" or hide the ability for subsites to create new subsites?

When a new user goes to register an account on subsite1.mainsite.com it is showing the "Yes, I'd like to create a new site" option to them. I would like to only allow them to register a new account. But if they go to the main site it will give them the option to create a new site.

Thanks.