If credits have already been assigned to a booking there are a couple if ways of removing the credits from the bookings;

Option 1:

If the booking is cancelled the default behaviour of the system is to add the credits back to the customers account (unless you select the option for the customer to forfeit the credits on the cancellation screen).

Option 2:

When viewing the Daycare booking, if you click into the Admin link from the left hand booking menu,

If credits have been applied to the booking already, the following option will appear on screen; Once you click on the Clear Credits button the credits will be removed from the booking.