Protecting your map from modders

What this guide is about:
A fellow mapper contacted me and asked how to lock down a map so modders can't open it, modify it and then re-release it as their own work. Our main concern were mostly inexperienced modders who just made minor changes.
So we cooked up some measures against this.

These measures do NOT protect your content (Textures, StaticMeshes, etc.) from theft!
These measures also do NOT make it impossible to mod your map! There is NO 100% secure way of locking the map!

These are just some hurdles that may or may not scare off a (newbie) modder.

Hiding your map
Select everything and set it to Advanced -> HiddenEd=True. This hides everything in the editor but not in-game. The modder just has to select all and change it back to False to make it show up again though.
You can also put everything in a group (Object -> Group. Type something like DontEditMe or something so maybe honest folk who just didn't know better stop when they get to this point), open the Group browser and uncheck it to hide it this way as well. If the HiddenEd setting is restored things will still be hidden because of this. And vice versa.
You can leave some things unhidden to throw people off some more.

Apart from hiding stuff you can also make the map painful to edit.

Leaving a ghost-map behind
You can delete all the brushes after the final rebuild. The geometry is still there, but they can't be edited anymore (except with Intersect/Deintersect) and if the modder has to do a rebuild everything will be gone - including the terrain and everything as it requires the huge subtraction cube to exist.

Boobytrapping your map
You can also craft a brush that is guaranteed to cause bsp-holes.
Read this. I recommend the inside-out brush from the 2d shape editor. Make it complex with lots of angles too so the map gets cut up nicely when it's added. Add it in another map, then copy it there and paste it into your map after the final rebuild. It won't affect anything until another rebuild is performed. Hide it from view by setting HiddenEd to True. Should a modder need to rebuild your map the brush will come into effect and litter it with bsp-errors.

Those should be pretty tough nuts to crack for potential modders. Just be sure to keep a regular copy of the map around on your hdd in case you want to edit something yourself later on.