Actually, during the Hybrid Madness offer period, you cannot remove the registration from your account yourself.
You have to request from Arturia to transfer it to the new owner (if you are reselling it), together with the free software (again, the free software instruments are linked to the master serial number and you are not allowed to un-bundle them).
The only way to return/resell software that has already been activated is to transfer (not delete) the licenses to a new USB-eLicenser (Steinberg Key) and include that dongle with the keyboard.
New Hybrid Madness licenses will NOT be be issued for the same master serial.
@Kim, if the store gave you a full refund without requesting the licenses back as specified, it is their mistake. But you could do the right thing by transferring the Analog Laboratory license to a Steinberg key for them to be able to resell the full package.
(no issue with free instruments, since you had registered it before the Hybrid Madness promotion)