As Coalition Honor Guard was card of the day on DailyMTG the other day, it got me thinking. If you gave it hexproof, through whatever means you choose e.g. Swiftfoot Boots, would my opponent be unable to play spells that target a single creature?
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.