You do this by going to 'settings' in the dropdown menu next to your name in the top right corner of the app.

Highlighting the user you want to disable

Clicking the 'edit' button

Unticking the 'enable' button

Save

If you ever want to enable them you can click the button under 'users' that says 'Show Disabled Users' and go through the same steps above but tick the 'enable button'