How do I delete unwanted event in Calendar app on my iPhone?

I recently had an event request show up on my calendar. It says: “Take advantage Today: your $500 Welcome bonus is Ready. It appears to be from HighFlyerCasino Promo and the sender is listed as *** Email address is removed for privacy ***. They have set it to recur from 12/15 through 1/15. If i open the request (which does include a link that i have NOT clicked on), it has the options of Accept, Maybe and Decline (as an actual meeting request would). If i chose Decline it will dissappear for a second and then shows back up again exactly as before. I cannot delete it, by doing anything. It is not from an unknown calendar. It is on the calendar that i use every day. I have checked settings also. (no unknown calendar or account). I have also gone into my outlook app and my Icloud and can’t understand why it is showing up on my IPhone Outlook calendar but not any where else. I am an Apple user and only login to my outlook using Apple devices. How do I get rid of these requests? This is extremely annoying and takes up a lot of room on my calendar. Not to mention that i'm concerned that the link could be clicked on in error or it’s a hacker. I can’t even report it. Thanks!

[Re-Titled by Moderator]