Once simple way is proof of work. There's a NIP for that. You can have the client mine the event for like, 1 second, 5 seconds, whatever. If it runs in the background people won't even notice, but it'll slow down spam. However this doesn't solve the problem of people throwing up random non-existent reports. But what if you just used consensus instead? This only applies once there's a serious network effect, but you could only show items that X number of people have reported/agreed exist. That's probably what waze does anyway.