You need an cryptor that will protect your files, for example a smart crypt ( I used it to protect some of my files ), can protect not only *.dat files, but also all the rest of your client, including *
ini and so on.
But be careful, it is still possible to get files from the SmartCrypt, but in any case it is a good protection. There is no perfect protection in the game.
Also, *.dat files can be protected manually too ( not SmartCrypt ), this method has been known for a very long time, but little practice, but in principle it works.