Deactivate a user
From the Dashboard, go to Settings > Team. Click on the icon (or picture) over the name of the active user you wish to change.
In the overlay either check or uncheck boxes to change the user's permissions. Click 'Save' to save the changes
To deactivate the user, click the red Deactivate User link
Reactivate a user
From the dashboard, go to Settings > Team, scroll down to Deactivated Users. To the right of the user's name to be reactivated, click the red Reactivate User button.