How to make a locked map?

Does anybody know how to make a locked maps for Garmin Gps?

I want to produce my own map by creating .img files and give it to some friends, but I don't want that they redistribute it.

I know how to create .img files, but I want to lock them.

Can anybody tell me where I can find a tutorial about it, or which software do I need?

Somebody told me about GMAPTOOL software, but I couldn't make it lock maps.

Thanks

Maury