Deactivating / reactivating a user can be done using the inactive mode
1. Click on the gear icon at the upper right.
2. Select User Management.
3. Click on the user profile you want to deactivate or set to inactive mode.
4. Use the toggle button to set user to active or inactive.
Then click on the save button.
Below is an image of user-profiles with the red profile deactivated.
5. To reactivate or re-enable a deactivated profile, click on the user profile and shift the button.
Then click on the save button.
NOTE: By default the user management screen only shows active users. To view inactive users you need to check the box for inactive users.