You can avoid downloading password-protected RAR files from Usenet by using an NZB search engine with curated listings. That means that each NZB listed has been reviewed by a human editor who will not list password-protected files. An example of such a site is NZBsRus.
Some purely mechanical NZB search engines like Binsearch.info can sometimes tell you if a is password required, which is great. However, it’s still better to have a human do the listing because he will evaluate other things such as video quality, etc.