To unsubscribe, simply log in and go to the Account Settings section of your profile.
At the bottom you will see the option I would like to delete my Domestika account. You will be asked for confirmation once you have clicked on it.
Once you have unsubscribed from Domestika, all your profile information will be deleted as well as the courses you have bought, to which you will lose access. If you have made comments on forums they will be maintained, but will no longer be associated with your name.