User accounts are free and will be forever free.
Subscriptions are paid on a monthly basis and payments are not automatic. So if you need to cancel your subscription all you need to do is not paying for a plan.
If you stop paying for a subscription your account will be automatically downgraded to a free account. All buildings which do not comply with free plan specification will be frozen.
Frozen buildings appear when a paid account has been transferred to a free account. Those are all buildings which do not comply with free plan specifications. Your data will remain for some time, but this building will not be shown or could not be edited. After some time, all frozen buildings will be deleted forever. You may unfreeze all of your buildings after you sign for a paid subscription plan again and use our "Extra" feature to unfreeze all of your buildings.