When you need to pause signups for a whitelist for any reason (prior to it opening or after), you can quickly prevent users registering by using /configure-whitelist close. While the whitelist is paused, it can still be seen by you and your community, but no one can sign up, even if they meet the requirements.
If you need to reopen the list at any point, the command /configure-whitelist open will allow you to do that.
Pausing/closing a whitelist that is already closed has no effect. Closing a whitelist also does not prevent you or any servers you have shared the list with from downloading the signups up to that point in time.