Sorry, all I did was respond to the original poster, @JiPi. If you are in a similar situation, you should contact product support. However, you are probably better off pursuing a refund from the seller (or asking him to cancel service and pay his bill so you can reactivate the device).
To be perfectly clear, YOU can perform the factory reset on the device itself. But that's only half the issue. The other half MUST be handled by the seller. He/she needs to remove the device from the active account.
Please review our Support Center article, Why is My inReach Device Locked?, for more information and options. In most cases, the best thing to do is contact the previous owner and ask them to remove the device from their account. If that isn't an option, request a refund. From the article:
"As these are potentially life-saving devices, we cannot cancel the service without speaking to the registered owner."