Deleted user still able to access Smartsheet

Hi, I'm looking for some help on how to eliminate some risk when team members leave the company. They are deleted as a user and sheets transferred, however these said users after they have been deleted are still able to access into Smartsheet when they log on with a password but not single sign on. Once logged on they can't see any sheets but, I'm wondering if there is a way to completely remove them and remove this risk completely?