They keep what they paid for. Everyone whose access period is running — on your catalogue or on one of your collections — keeps reading everything they could read, until their access period ends. That holds whatever you do afterwards: withdraw works, take articles out of a collection, empty it, stop publishing, or leave BSVSearch. By selling an access period you licensed BSVSearch to keep serving those works to everyone who paid, until the last of their access periods has ended (Creator Terms, section 3).
Withdrawing stops new sales, including new subscriptions. Somebody who subscribes after you withdraw a work will not see it.
Nothing you do pauses an access period. Only a rights claim, or BSVSearch taking a work or collection down, does that: the clock stops, and the time is given back when it returns.