There's no way to 100% protect the map except by corrupting it, which will make it unplayable.
Someone's going to unprotect it some time so they can add cheats. Just because they suck at the map. It's ok. Just make a new version. What I'd do is make it so that if they try doing the cheats, display a message like "CHEATER DETECTED!" Then kick them.
Give link to official map download and encourage people from downloading there. It' s good news that your map got so popular that someone thought to edit it.
Follow these steps to ultimate victory:
1.) Calm down
2.) Say "NO" to drugs
3.) Do well in school, make healthy, good friends
4.) Work on your own version of the map, and change things a little... well a LOT different from the hacked version, tell everyone that the hacker version is a fake and that yours is the REAL TRUE NOT WRONG one.
5.) Do this step before step 4 =D - is it even hacked, maybe they exactly copied it and messed it up... it's just a name.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.