Coalition Honor Guard and Hexproof
Your opponent can still cast spells.  As the last part of Coalition Honor Guard's text says (emphasis mine):


  your opponents must choose at least one Flagbearer on the battlefield
  if able.


If they can't target a flagbearer (because of, say hexproof or a restriction on the spell, ie. "target green creature" and all flagbearers are white), you don't have to obey the rest of that text.