RevMob: Question about Placement ID's

What is the correct way of enable and disable ad units via placement ID’s?

I can disable an ad unit if I remove the placement ID in the revmob dashboard, but how to enable it again properly?
[import]uid: 106768 topic_id: 35273 reply_id: 335273[/import]

No one? [import]uid: 106768 topic_id: 35273 reply_id: 140450[/import]

No one? [import]uid: 106768 topic_id: 35273 reply_id: 140450[/import]