While there isn't a way to delete an affiliate user, you can go into their profile and change their commission to 0%.
- From your dashboard select Affiliates.
- Once in, select Users and click the Edit Pencil next to the affiliate you want to change to 0%
- Once in, scroll down to Affiliate Percentages. Choose the Percentage Override you want, make sure to click the checkbox next to "Allow Affiliate's Percentage To Be Overridden" and click Save on the top right.