Saw the question-mark in the upper-right fading in and out indicating a calendar event. Turns out to be a real-money site (GuildBoost.net) sending spam calendar invites. Please add an option under Gameplay/Social to block calendar invites the way we can block guild invites and block trades
this isn’t a bug.
if you want to make a suggestion, post in General Discussion.
(and leave out the advertising for the gold selling company) 
but you’re right… it makes no sense to not be able to block calendar invites.