The reason for this, is here is no expiration date available on Events like there is on other content types.  Because events simply only have an event end date, it won't remove them from google search because they still exist in the database even though the event has passed, it's still physically published as it lives in the Event Category directly. In the meantime, to get those events to not display, you could delete those old events OR make the Event Category secure/private to a Role such as Platform Manager (which means if you are still actively using the Category, it would be best to create a new Category and move the Event into the new Category.  Then via Google Webmaster tools, request a recrawl of the site.  We are not sure that privatizing the Event Category will entirely do the trick, so deleting would be the cleanest/easiest/ most efficient. option.

A similar request has already been made in the new Ideas portal on the K-12 community to expire past Events.

You can see the idea and subscribe to it for updates here:

https://blackbaudk12.ideas.aha.io/ideas/K12CO-I-1026

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Feel free to add comments to this idea to provide any more relevant details.