Anything you put online will eventually be taken and used without your permission. But as Ascension and mearrin69 have said, you can easily take steps to protect yourself legally.

Personally, I stopped worrying about it a while ago and uncopyrighted the majority of my maps (unless a client paid for them), so anyone can use them for whatever purpose they want, be it personal or commercial. That path certainly isn't for everyone, but I guess my point is that sometimes we just need to let go a little and focus on why we make maps. For most of us, the benefits we receive from sharing our maps, getting constructive criticism and learning as a community far outweigh the potential risk of some schlub stealing our content and claiming it as their own.

If you're worried about it, I suggest you put a license notice in your signature (Creative Commons is a popular one) so anyone taking your work has seen explicit reference to your ownership. That, coupled with your ownership of the source files, should be enough to protect you legally.