Have you been receiving some suspicious calendar invitations recently? Most likely, those are spam messages. Getting distracted with meaningless push notifications can be really annoying. However, the fix might be easier than you think.
Here is how you can get rid of this ->
1. Go into your Settings app -> Passwords & Account.
2. In the 'Accounts' section remove the account(s) that do not belong to you, or that you do not recognize.
These two steps should resolve the problem.
Quick & easy, right?:)