Team members are not allowed to quit their teams on their own. The team's administrator will need to make that change from within his or her own account.
To do so, sign into the administrator's account (or ask them to) and click on the Team tab on the left side of the Dropbox Fax window. Find the list of members towards the center of the page, and then the email address of the person who needs to be removed. Click the small downward arrow to the right of their address to pull down the menu. "Remove" will be the last option.