1. Install Snitch & activate: https://wordpress.org/plugins/snitch/
2. Open the settings/update/licensing page of the plugins to block.
3. Go to Snitch. You will find all the external request that has been made from previous plugin here.
4. Click “Block host” and “Block file”
5. Done! This will block any external http request from the plugins which normally being used to connect to their server to check license, updates.