If a sub-block is set to have access until X date. After X date will they be allowed to make modifications?

At the end date of the "Active Dates", and if the "Active" is checked on, guest can still access the sub block to cancel their reservations only. They can not make new or modify. If you want to allow access to inventory for a longer period of time, you will need to change the active end date. If you want the sub block to be completely inaccessible, you will need to unselect the "Active' status.