You can apply for cancellation of GST registration online. After the Officer is satisfied then the reg can be cancelled in 30 days by Form no. REG- 16.
The steps are:
1. Login on the GST Portal - www.gst.gov.in with your user is and password
2. Go to Registration> Application for cancellation of Registration.
3. Fill the address for correspondence in basic detail of the form.
4. Go to Cancellation details> select reason for cancellation> select the date from which the reg. is to be canceled> ebter other details.
5. Go to verification details> submit with EVC if not company/LLP.