A reminder can be created in various ways; job reminders, invoice reminders and manually created reminders through the CRM.
In the Web Portal, go to Admin > Reminder Logs, and you will see a list of reminders that have been done or are scheduled to happen.
Scheduled reminders can be edited individually or selected and deleted individually or in bulk by selecting the checkboxes > Delete at the bottom of the screen.
You can also delete the reminders for a specific client by going to CRM > All Contacts > select the edit arrow for the contact > select Reminders. There you will see a lit of future as well as past reminders relating to the contact.
To delete them, select the checkbox(es), and then select Delete at the bottom of the page.