Yep … they’re annoying aren’t they? Blizzard were apparently working on something that might block them: https://www.wowhead.com/news/new-option-to-restrict-calendar-invites-datamined-on-the-patch-10-1-5-ptr-333376 but so far nothing seems to have come of it.
To report them you have to exit that screen, and on the main calendar month-view screen, right click on the event and choose the report option. I don’t think it matters greatly which option you choose, because (a) I’m not sure they ever get dealt with and (b) they’re usually sent by level 1 characters that are probably deleted afterwards.
In theory once you’ve reported it, it should disappear from your calendar … but either there is sometimes a bit of a lag before it happens, or you need to report a couple of times for it to be removed.