I don't think it's reportable unless they directly start harassing you-- or being disruptive. Even more-so since it's a map available to the general public. A friendly, polite whisper explaining that there's a roleplay in progress is the most you can do, sadly. I've accidentally bumbled into a few after downloading maps from deviantART. As you said, if they don't, you might need to encourage your group to block or ignore them.
Can definitely understand the irritation though; I once had someone burst into a private map-- which was a bit of a pain. Even so, I hope it all gets resolved!