The onus is on the previous user to make sure the device is no longer associated with their account, before selling/giving the device away. Then the new user can connect and activate the device as if it were new.
If you can't get the previous user to do that, then perhaps a Comcast rep on here could help.
Hi EduardAtX, thank you for your private message and update, glad you were able to get this resolved. Please reach back out if you need any assistance in the future.