The various pledge statuses are:
Active: When the pledge still has instalments that need to be paid.
Closed: When all of the instalments have finished.
It's also possible to close a pledge by entering an End Date on the pledge. Providing there are no active instalments the pledge will update to be closed.
Written off: When you used the option from the Manage menu and written off the pledge to stop payments from being taken.
If you go to Finance, followed by Pledges, you have the option under Manage to Write off a pledge. This can be done to any active pledge; including in bulk.
So, for example, if you wanted to write off 20 pledges at a time - you would need to get them into a yellow box search. Once all are selected, click Select all and then Manage and Write off. This would write off multiple pledges.