You may have some users within your Tenant that should never be shown within Teacher Dashboard.
By creating a Blacklist of users you are ensuring that they never be added to classes, or selected when sharing assignments or files with individuals.
Log into Teacher Dashboard as a Global Administrator and click on Global Administration from the top menu. Go to the Global Settings page and click on AAD User Sync.
From here you have the option to set up a Whitelist of users or a Blacklist of users.
To set up a Blacklist, ensure the first radio button has been selected: "Sync all users (except those listed below)" and start to enter the users email addresses in the text field using either a comma separated list, or enter each email address on a new row
When you click on Save, Teacher Dashboard will only show all users except those entered in the blacklist
At any point, if you need to amend the blacklist, you can add or remove users and click 'Save'