How to stop theme piracy ?

At least with your WordPress theme there may be no chance, as eventually every WordPress theme has to be GPL (or compatible) licensed. -> https://make.wordpress.org/themes/handbook/guidelines/license-theme-name-credit-links-up-sell-themes/

What you sell: Support, so don’t answer support request to anyone without a valid license key.

What else you can do:

  • Implement some licene mechanism, which checks against the generated license key from Envato
  • Make your theme better over time, so people want to buy