If you want different access restrictions to different parts of the Help Center. For example, if you want the setup to be:

1. Community is public - login isn't required to access this, you only need to provide an email address
2. Tickets can only be submitted by users who are approved manually

So to have closed registration for Tickets, but allow Community Topics to be submitted by any visitor to the Help Center you need to implement selective access. The way to configure this, where part of the helpdesk is public, but part is restricted, is with usergroups.

To implement this:

1. Go to Admin > CRM > Permission Groups, and remove ticket permissions from the "Everyone" and "Registered" usergroups (and any other usergroups). Make sure that the "Everyone" group has permission to use Community. 

2. Create a new "Approved" usergroup and grant it Ticket Permissions.

3. Add users who are allowed to submit tickets to the "Approved" usergroup. Any users who are not in this group, won't be able to submit tickets by email or see other parts of the Help Center unless they are added to the "Approved" Usergroup.