There are multiple ways to remove credits from a customer depending on how they were added, which could be; via the till as a one off product, or as bolt-ons.
Removing credits applied through the till
Refund the sale of the product, this would then remove all credits applied on this transaction.
To refund you will go to the members profile, purchases tab, then click view on the till transaction linked to the credit.
Now you can refund the sale.
You can book in the customer to an activity in the past this will then remove the credit added.
This would not refund any payment.
Removing a credit applied by a bolt-on
If the credit was applied by a bolt-on you can cancel the membership plan along with un-used credits.
Go to the members profile
Click on the membership tab
Click "edit" on the plan which gave the bolt-on
Cancel with unused credits
You can book the customer into an activity via the diary to remove a credit.