Hi,
Do you see the event directly in your calendar or also in your inbox email list?
In Outlook on the web, open Calendar, select the suspicious event, then choose Delete or Decline and do not click any links inside it. To report it, switch to Mail, find the related message, open it, select Report on the toolbar, then choose Phishing so it is submitted safely. To prevent more, go to Settings > Calendar > Events and invitations, turn off Add events from email, and set invites to appear only after you respond. This stops spam invites from being added automatically.