If the registration fee refers to an amount you are refunding because someone had originally registered for one of your programs, I would:

1) credit Cash for the amount you are paying out as the refund,

2) debit a contra-revenue account such as Refunds of Registration Fee Revenues. This will allow you to easily track the total amounts of refunds that you make during a year. On the other hand, if it is rare for your organization to refund registration fees, you could simply:

1) debit the amount you are refunding to the normal revenue account such as Registration Fee Revenues

2) credit Cash.