To disable a buyer you need to head over to the buyer section. To do this go to Campaigns > Buyers > Actions then select Disable to disable individual Buyers:
 

Alternatively, you can select multiple Buyers to disable together:
 

 If you want to delete a Buyer(s) you can select Delete instead of Disable after selecting the Buyer in question to completely remove them from the system.

If you want to remove a buyer from a delivery without disabling it, simply open the delivery configuration, unselect the buyer or change the buyer to a different buyer.