Open the native Mail app on Windows (10 or 11), find the invalid event in the Calendar (bottom left), and delete it from there.
I’ve been having an annoying problem lately, I tend to set reminders for recurring tasks in my calendar, why have something take up expensive space in my brain when a software can do it for me? However, I recently had an annual event that I no longer needed, so I removed it from Outlook. Even after removing the event, I was still receiving reminders for it on daily basis.
Every time I opened Outlook on desktop, there it was, every time I had another reminder, there it was again right beside the legit reminders. I couldn’t snooze or dismiss it. Nothing worked. I tried the solutions mentioned here and here. No go.
The fix was unexpected, but very easy.
I simply opened the native Mail app on Windows (10 or 11), went to the event in the Calendar (bottom left), and deleted it from there
Weirdly enough I wasn’t getting reminders on my phone, or from the web app either. The annoying pop up was only shown by Outlook desktop application, but it’s gone after I deleted it from native mail app’s calendar.
Hope it helps.