Honestly a convention should be the easiest place to get evidence.
You would think, except we have a whole lot of social inertia and misconceptions that get in the way.
There are so many people it would be very hard to harass someone for very long without someone noticing the behavior.
This is the good thing about a convention - it's not actually all that big. While GenCon and ComicCon are huge, most conventions are just a few hundred to a couple of thousand people. That means tracking reports isn't all that hard. In the case I spoke of earlier in the thread, where a lady I know got groped, after talking with her, she allowed me to report the incident. Now, the convention didn't act instantly on my report. I didn't even know the guy's name, but I had a description. They put that together with the several other reports they got about the same person, and that established a pattern of behavior, and they permanently banned him from the convention.
Bad actors don't generally restrict themselves to doing only one bad thing for an entire weekend. Especially if they don't realize that they are getting reported - they *think* they are getting away with it, so they continue, and leave a trail of events. And that's something the convention can notice.