Yes, it is possible to exclude a user from the affiliate program in Magento. To do this, you can follow these steps:
Log in to the Magento backend as an administrator.
Navigate to "Customers" > "Manage Customers".
Find the customer you want to exclude from the affiliate program and click on their name to edit their account.
In the "Affiliate Information" section, uncheck the "Is Active" checkbox.
Click "Save Customer" to apply the changes.
This will deactivate the customer's affiliate account, and they will no longer be able to earn commissions or participate in the program. You can also use this method to re-activate a customer's affiliate Magento account if you have previously deactivated it.