Spammers using tricks to prevent filtering words in subject line.

I don't think Cox even has spam detection any longer because it no longer detects any spam that even my old Windows Live Mail can detect and place in the junk folder.  So I have been trying to use the filters to prevent downloading of spam to my email client.  Spammers use a different email address each time so blocking or filtering the "from" won't work. The body is either in HTML or a photo with the ad so nothing to filter there.  So I was hoping I could filter out by detecting certain words that frequently appear in the subject line.

Well, seems that is useless as well.  For instance, I see the word military in the subject line often so I set a rule to filter any incoming emails containing military in the subject line.  Well that doesn't work because it seems spammers can use some tricks to prevent this.  Although the word appears to be "military" spelled correctly, when I forward to Cox spam report, the spell checker in my email client flags that word and every other word in the subject line as spelled wrong.  It looks something like this in the forwarded email:  m_ili_t_ar_y.  Every other word is broken up with either a _ or ? as well, although in the incoming email it looks perfectly normal.

Can someone explain what is going on and is there any way to defeat this trick?