If you want to deactivate one of your listings, you can change the stock of this listing to 0. This will set the listing offline.
Note that you can't delete an offer, but you can archive it to clean your catalogue.
Once your listing is archived you can see it back easily by clicking on "Archived".
If you want to push the offer live again you just have to click on "Publish offer".
If you want to deactivate a lot of listings, you can send the list of the corresponding SKU to firstname.lastname@example.org.