As I've proposed in the previous flag-lock thread, I'd prefer flag locks to disable a flag for their duration in addition to locking them (and have a slightly longer duration unless that proves unbalanced), while being able to be used on all flags, own, neutral and enemy.
Locks serve a purpose that would be partially broken with a shutdown mechanism, but it could work as a seperate item.
The Flag Bag: 350 gold, short channel, disrupts flag effects for 1 minute. I could see this having the most benefit with portals.