We don't automatically disable live sites when an activation becomes invalid. Just in case users have moved servers, we don't want to cause any unnecessary problems... they will just get an error displayed in the admin stating the activation is invalid and instructions to contact us for another license.
You can clear the activations on our downloads page directly. Just do it immediately before you are activating it on a new domain and you can take over the activation... then they will start getting the error message in their admin.
If I were to clear it before then (like now) it would just automatically re-activate the next time they accessed the site.