Is there a way to desactivate/unpublished an offer automatically?

Under schema nms:offer there is a Cancel operation 

It takes 3 arguments namely

offerId

comment

offerSpaces

Pass in the ID value of offer and the spaces from where you wish to deactivate the offer along with a comment.

From within JS of Adobe Campaign you can call the same function. Kindly refer to https://experienceleague.adobe.com/developer/campaign-api/api/sm-offer-Cancel.html

Please make sure you take necessary action on propositionRcp records which are in either Presented or Activated status. You might wish to cancel their status as well.