1. Application Management -> Site Collections -> Configure Self-Service Site Creation
2. Security -> General Security -> Configure Self-Service Site Creation
Select "Off" in the Enable Self-Service Site Creation checkbox.
If set to "On", then the users who has the "Use Self-Service Site Creation" permission will be able to create a top-level site collections.
If set to "Off", only Farm Administrator can create top-level site collections from Central Administration. Note that in this case Power Users still can create subsites under the site collection/parent site.
Note: Self-Service Site Collection creation can be performed from http://siteurl/_layouts/scsignup.aspx if this opiotn is enabled.