You can turn this option on or off at any time by following the instructions below:
- Login to the administrative side of your site.
- Go Setup > Site options > Site Settings > Allow Signups.
- Turn this option off by clicking the check box so that the green check mark is not showing.
Note that shutting down registration is site-wide, not specific to any age group or challenge. Those who already have a login are still able to participate in current challenges on the site and login as usual.