Sometimes people have to quit a job and it's no use keeping them as Workspace members anymore. As an Administrator, you can deactivate your workspace users. Deactivated users will not be able to open your workspace, access will be denied.
If you want to deactivate a user, follow the steps:
Hover your cursor over your workspace name in the channel tree of Leverice on the left;
Click 3 dot button which appeared to the right of the workspace name;
Click "Manage Members";
Hover your cursor over a member name;
Click on Active toggle button to deactivate a user.
The user is deactivated now!