You can only delete offers that are in Draft. Once the offer is Active, it can't be deleted.
However, you can still prevent the affiliates from sending traffic to an offer if necessary. To do so, navigate to Offer detail (open detail of any Active offer) and click on Deactivate offer button.
Once the offer is deactivated, affiliates can't access it, and you won't need to pay for conversions generated after the offer's deactivation.