You could allow the user to flag it and keep the alerts to only them. They would get the alert, they could add notes of what happened so when that plate shows again they will know. Maybe build it so that you can add friends to a group and share plate details. It's GOLD Jerry!

But yeah moderation would be a nightmare still - I could see a ton of ways to abuse this. Criminals will figure it out as a source to track individuals, placing themselves in their known travel path, setup satellite spots to alert, that way for example if they're casing the joint they could have alerts when then individual is coming back.

Great, now ppl are going to make this app and use it for bad, see what you did.