In order to remove someone from the list of users who are accessing applications owned by your billing account, the billing account manager should add the user to the deny list. Denying a user ensures that the user can no longer access any application in your billing account. And, you are not charged for users who are on your deny list.
This error indicates that you've taken up all the storage space for a particular table. In order to keep the "quick" in QuickBase, tables have a maximum size limit. To see how close you are, you can check your application's statistics: