Surrender of service tax no.

You can fill surrender application from ST-2 online, by logging to www.aces.gov.in.

After filling the form, take print & submit it to the Superintendent along with a cover letter about reason for surrender etc. Then, he'll check your books of accounts, returns, challans etc. to make sure that there no service tax is pending to be paid from your side.
Then your RC will be cancelled.