terminate Account

Which kind of subscription? If you mean forum email notifications, then you can disable it in your forum profile settings. If you want to disable just notifications from a specific thread, you can do it at each thread separately (click More under the opening post).

If you want to cancel your Garmin account completely, you can delete all data and close the account at https://www.garmin.com/en-US/account/datamanagement/ 

Your Garmin subscriptions can be handled from the subscriptions page in your Garmin Account:

https://www.garmin.com/en-US/account/subscriptions/