Reproducing/modifying is not allowed if you specify it as a copyrighted work. That's what copyright means All you should have to do is add one of these nice little (c) symbols along with your username (or your real name, if you wish) and the year. I'm sure you know how to do that. If you still feel uncomfortable, you could add a license to the description (i.e. in the forum post, next to the link) of your map. There are a lot of licenses out there, the one you're looking for would be

It only allows others to download your work, not change or redistribute them in any way. You can, however, take a look at this page as there are some other licenses to choose from as well. For example, you could say others are allowed to do anything they want with the map and even redistribute it, however it has to be non-commercial.

Keep in mind that I'm not a copyright "expert" in any way but I am pretty sure this is all you have to do.

Cheers.