To deactivate your License Key, just log into your website then navigate to Stackable > Account in your WordPress dashboard.
Once in there, click on Deactivate License.
You can also log into your account in wpstackable.com and perform the License Key deactivation from there.
Once logged in, just head over to Websites from the menu.
Click on the website you want to deactivate your License Key on, then on the details pane that opens up, click on the Deactivate link to deactivate your License Key from that site.