You can turn this option on or off at any time by logging in as a library administrator.
Get there: Setup > Site options > Site Settings > Allow Signups
Toggle this option off to turn off the ability for participants to register for your Beanstack site.
Note that shutting down registration is site-wide, not specific to any age group or program. Those who already have a login are still able to participate in current programs on the site and login as usual.