Edit: Finally got the seller to deregister. Thank you for the help.
Bought a used hub and I asked them to make sure they wipe the hub and deregistered it. They said they did and I asked again to confirm in which they told me. Yes it was already done.
Well they didn't do it and they can't make any changes to the hub and I can't register it.
For de-registering you will need support from Hubitat.
To clear the Hub itself, there are multiple threads here on the forum describing soft and full resets, either via the UI or using the reset button on the back (small square hole).
There is also an obscure factory reset option.
If you need help with resetting your hub, let us know, there are many forum members who can help here, just the de-registering from the former owner is out of out reach and there you need either @Hubitat_Staff. Tagging @bobbyD as an extra