Hey there, thanks for getting in touch!
A fast way to clear venues and organizers is by removing them from the Venues and Organizers options under Events in your WP admin menu (screenshot).
You can remove each venue and organizer individually or delete them in bulk by checking the boxes next to each name and selecting “Move to Trash” in the Bulk Actions drop down. Once moved to the trash, they will no longer be available as options when creating a single event.
Does that make sense? Will that work for you? Please let me know. Cheers!
Geoff