Rogue calendar event
Your Calendar must be set to automatically accept invitations from email.
CHECK FOR THIS SETTING:

Automatically retrieve CalDAV invitations from Mail
Select this checkbox to have Calendar get event invitations from Mail.

Go to Preferences in your Calendar and turn it off. (for now)

Next, set junk filtering in your Gmail.
Also block this specific user.
Just to make sure, change your Gmail password.
Clean all Internet saved browsing (the history) including cookies.
Then run a virus scanner to finalize.
Once you are sure you got your Gmail account under control with decent junk filtering, you can turn the auto Calendar invites on, if you wish to use it.
If there is a leftover from previous owner, check the Delegation set up in Calendar, by clicking on the EDIT. There should be no names in there.