You can disable and re-enable users in your Workspace by going to Settings > Workspace Settings > User Directory.
To disable a user, click on the red "NO" symbol next to their name.
To re-enable a user, go to the Disabled Users tab and click on the Recycle icon next to their role.
Note: A disabled user will appear here even if they've been removed from the Organization by the Owner. If you try to re-enable a user who's been removed by an Owner, you'll see this message:
In this case, an Owner will need to re-enable the user in the Manage Organization and Workspaces Dashboard, and the user will need to be re-invited to the Workspace.