Go to the Activation screen of the current license you need to press the Deregister the code button.
But if you already remove the plugin without stopping the activation you should not worry. Login to the managing of license screen and manually deactivate all past installations that you see with the button on the right. To view existing purchase code activations you can press the Manage Activations button from the plugin screen or directly visit this address http://activation.socialsharingplugin.com/manage/
To log in, you need to fill your Envato username (in the same way you type it) and the plugin purchase code.
When you are logged you can press deactivate the key from any active website.