Sincde a couple of days it is possible on the GST portal to cancel the GST registration for dealers who are migrated because they were registerd in the previous Commercial Tax Regime (VAT, ,LT etc) but have an annual turnover less than Rs. 20 lakh.
Login on the GST portal and click in the right site of the page on: <Cancellation of Registration of Migrated Taxpayer>
You get an OTP and later an e-mail acknowledgement of cancellation with the warning: "In reference to the above application, your provisional ID is cancelled effective from appointed date. The application is subject to verification by the jurisdictional tax officer and an appropriate action may be taken by him."
Good luck.