Penalty payment

I have a client who had a service tax registration in the year 2004-05 but after 2 or three years he cancelled being not necessary any more in the business. 

After a cancelling the registration he got a letter for paying penalty of Rs.400 to the department, now as he has cancelled his registration from the department , how to pay the penalty of said amount and by which name it should be ?

please answer soonest possible.. 

Thanking you