Under the People tab on the left hand side you will search for the person you want to unassign a fob from.

Click on the person

Once the page with their information has loaded you will look under the section labeled Key fobs and see the key fob assigned to them. Click the 3 dots on the far right hand side and click unassign.

To ensure the fob's access has been removed you will need to update key access manually at the doors.