If you want to disable registration in DNN and only want the registration to be private. You can easily do this by login as Site Administrator.
Once login, in the left navigation panel and under the Settings menu there is a menu option called Security.
If you click this option. Under the Member Accounts, there is a registration settings tab. In the User Registration option, just tick the Private option. This will ensure no visitor can register an account on your DNN site.
Those two screenshots are taken under DNN version 9.