How can I delete spam calendar events on iPhone?

Is it that you have a hotmail, outlook, or other Microsoft Exchange email account. If so, that is the problem and the most reliable solution is to quit syncing that email account with your Calendar. Microsoft has had this problem for a very long time that comes and goes where their servers are infiltrated with spam events that they cannot control. Those events sync to your Calendar because they have been allowed to do that in the settings for the Account. Just turn off the Calendar for that email account, so it no longer syncs.

PhiloLefty wrote:

same - events no option to delete, no subscription to delete, can't decline ......

Delete calendars and events on iPhone - Apple Support