If the amount is not refundable to customer then amount is liable to service tax as forfeited advance is in nature of consideration for the service rendered and we would have already paid the service tax on it, no need to pass the separate journal entry.
If the amount is refundable to customber and tax is already then it is the nature of refund of consideration hence we can claim refund of service tax. Accounting treatment is refund the entire amount of payment received from the customer claim the service tax refund from dept or else if any other similar service has been provided to any other customer then we can consider that payment of service tax is for this service