I gots alot of ideas about this.

If someone attacks a player with magic or ranged, they could be set to fight as a mage/archer and won't be able to switch to a different combat style during combat.
This will then remove a problem with Sen's idea, where different combat styles have different flee rates. Someone could attack as an archer to get the high flee rate, then switch to using swords.
Maybe placing arena's where only one type of combat style is allowed, and another where all types are allowed.
   
     I'll post more once I figure out how to explain it.

May I be a Betawulf please?