Removing spam invite from iPhone calendar after declining

Hi!

I've tried all the solutions online, however, none of them work for me.

I stupidly clicked decline and now the invite is in my calendar every day until next year. I want to remove it ASAP.

These are the things I've already tried:

1. Cut method - doesn't allow me to paste on a new day
2. Check spam folder - deleted spam and junk folder, but invite still in calendar
3. Deleting unknown calendars - the invite appears to be part of 'calendar' in my phone, which cannot be deleted and also has other calendar invites associated with it (ones I know and have accepted)
4. Settings -> Calendar -> nothing odd showing there...

Can you please help?

Thanks :)