Commissions are tied to transactions. You can decline a commission from an individual transaction record.
To search transactions by affiliate, you can do an advanced transaction search under Sales & Transactions >> Transactions > Advanced Search. In the advanced transactions search, filter your search by the Affiliate ID field. In the transaction search results, click 'View' next to the Transaction ID. In the Transaction record, look for the "Commission" field. There you'll find a "Decline" link.
You'll need to do this for each transaction individually.
To prevent future commissions being awarded, you can remove the affiliate ID from the client record. While you're in the transaction record for the client, you can click on the 'See More Details Of This Subscriber' link next to the client name. There, you'll find a field for the Affiliate ID of the referring affiliate. You can set the affiliate ID to 0 and save.