Go to the Calendar section of eM Client and switch to the Agenda view. You will see the icon on the toolbar after Day - Week - Month.
Uncheck “Hide Past” option by clicking on that icon. You should be able to see all your past events.
Select all events for which you want to dismiss notifications.
Open context menu by right clicking any of selected events and chose Reminder > No Reminder
You will no longer get reminders from past events.