In the Top level site  of the Site Collection you want to go to Site Actions >
Site Settings > Advanced Permissions

From the Settings Menu select permission  Levels. Add a Permission level that
contains all Permissions except create  Sub Site Permission.

Go back to advanced permissions and access the Site Owners Group. Uncheck the
Full Control permission Level and select the new Permission Level you created
instead. Make sure to do the same with anyone else who has Full Control since
you can't delete that Permission level.

Once complete only Site Collection Administrators and those who have Full
Control can create Sub sites. Just remember that others can manage permissions
and may re-assign Full Control since you can't delete it.