Appointment notifications won't go away

I have the Timetastic Staff Leave Planner schedule synced with my eM Client calendar.

I keep getting reminder notifications popping up in eM client about an upcoming leave which has been approved and whatever combination of buttons I press (dismiss, snooze, dismiss for 2 weeks, snooze for 2 weeks etc.,) the notification keeps popping up.

If I delete the appointment from my eM Client calendar it just recreates itself seconds later.

What is happening here? How can I make it stop?

I have already raised the issue with Timetastic who say:

"This looks like an issue with the way eM is handling the iCal feed through Google.  Our calendar feed adheres to the standard and works with Google, Apple and Microsoft calendars but some clients treat the standard differently.

We’ve not heard from any other users using eM, so I can’t say if this is a known issue with their software or not.

I’d suggest getting in touch with their support with this one and hopefully they can offer a solution."