You can refund a participant (and revoke their access to the course at the same time) via the manage → participant list page.
If you haven't granted refund permissions for your Stripe or PayPal account, you'll need to do that first. You can find this option by going to the account drop down menu at the top of the page, choosing account settings and then picking payment gateways from the left side bar.
Refunding Stripe Payments
For Stripe payments there's nothing you need to do - it just works!
Refunding PayPal Payments
PayPal requires an extra step. After connecting your PayPal account you'll see a button to grant refund permissions to CourseCraft:
Click that button and you'll be taken to this page on PayPal:
Click Grant Permission and you're done!
Note: PayPal only allows refunds for 60 days. The refund button will not be available if the participant has been enrolled for too long.