Dispose Chromebook

Here is my issue, we have multiple sites and bring back old retired or damaged devices and then send them to the recycler. When I mark the chromebooks as disposed after google sync it changes their status back to deprovisioned and assigns it back to the last student that used it (as long as their account is active) as well as the original device. I think I figured out the deprovision issue but cant seem to find out how to stop the assigning on retired devices and moving back to the original site. Any thoughts or suggestions would be awesome.