Hi,
Today I don't see any way of disabling a content so that it doesn't appear in Campaign, apart from deleting it, which is obviously not the thing to do!
You can easily filter contents so that contents that are already synchronized to deliveries and/or unapproved contents don't appear in that list, leaving only approved and unused contents (which should be the newest ones as oldest contents would probably be used already). Unfortunately, if you have hundreds of approved and unused contents, they will all appear there.
Note that contents that are not approved in AEM can be synchronized to Campaign deliveries but you won't be able to send the delivery.
Hope this helps,
Florent.