Helix ALM (aka TestTrack) only send email notifications for rules configured in the system. There are two places where these rules are configured:
(1) Automation Rules - The administrator can configure email notification rules or escalation rules to send email to anyone. Generally this contains generic rules such as emailing a person when an item is assigned to them.
(2) Notifications in User Options - An individual user can configure email notification rules to send email to themselves.
I would suggest looking at these two locations to find which email rule is generating the specific email. If it is a "reminder" email, then it sounds like the escalation rules in the Automation Rules dialog. Once you find the specific rule, you can either delete the rule or put additional restrictions on the rule by specifying a filter in the rule (ex: only for issues in an open state).