In theory, this should stop people from connecting to your server, but upon testing, they can still connect to the TCP port, they just can't send or receive any data from the server
but still, use at your own caution.
WEB UI STEPS
On the left hand tab, click mission
you will see the name of your user with a check box that says "is private". you will need to deselect it to broadcast it to your server. either do one user at a time, or ensure everyone knows their assigned name in order to download the appropriate data package with their credentials.
after the user downloads their credentials, make it private again.
ATAK STEPS
Click the top right menu
select data packages
click the box with a down arrow in it
click the server
you will now see your credentials. click on it and select download
go back to your server list again using the steps from the previous page and you will see the SSL server
deselect the server that says TCP, and select the one that says SSL (will say :8089)