Jonathan,
Thanks for getting in touch! Yes, you can hide the Organizer/Venue dropdowns, but unfortunately the form will treat any future submissions as new Organizers/Venues. If you want to hide these dropdowns, take a look at this thread.
Regarding avoiding duplicate entries, unfortunately we don’t currently support anything like that, but it’s a great idea! Feel free to submit it as a feature request on our UserVoice page to be considered for inclusion in a future release.
One thing that might help would be to display some instructions at the top of your form, instructing users to select existing Organizers/Venues before entering new ones. You can do this using this tutorial.
Hope that helps! Thanks! 🙂
-Casey-