You can't. You could stop mIRC from opening the files internally by butchering the toolbar but the user can still open the script files in any word processor. (Notepad, Wordpad, Word, Wordperfect, etc)
There is also no way of scrambling the code because mIRC then will not recognise it. You could write a DLL file (as suggested by users replying to similar questions about rip protecting scripts) but that requires a knowledge of a programming language. In my view though, this is a complete waste of time. I, as a personal option, am proud to put my scripts on display so that people know they are:
1. Trustworthy.
2. Reasonably efficient.
If someone rips your work then consider it a good reflection on the quality of your work and a bad reflection on the integrity and knowledgeability of the ripper.
