Apologies for the delay in a response.
Firstly the installers shouldn't have an issue with de registering, even if its registered with an installer rather than your own email.
Unless the system does not belong to you, ie rented or on some kind finance.
Login into the Pyronix cloud on a computer, and you should be able to request a deregistration, if you are unable to find that option you can opt to raise a ticket at https://www.pyronixcloud.com/ticket/
Its is important to note that Pyronix will email the account holder and ask them to verify the deregistration, if there is no reply then you may have to wait a few weeks before the panel is released.
If you were transferring to us, then we could check on the progress and advise on the next steps to take.
Unfortunately it can get very involved and each circumstance is different.