I also like the trick about ready and action that would draw an AO on the characters turn to avoid the AO. Will have to try that.
This is an annoying tactic (out of game), and I'd use it sparingly. It's annoying because initiative order changes, which is a hassle. If you do use it, make use you're being fair about the ready trigger - you need to trigger on something concrete, so you need to predict what your opponent will do, which isn't easy. Secondly, it's a reaction, so if your opponent is nasty, he might take an action that invalidates or makes less attractive your action.
There's also a fundamental imbalance here, which a DM should definitely not abuse: you as a DM know what the PC's are triggering their readied actions on, but you might feel it fair to keep your triggers secret. That makes for a metagame imbalance which you really want to avoid. Personally, if this were to come up for me, I'd at least announce that the critter is holding back waiting for the right moment - let the PC's note the delay/ready. Perhaps a perception or insight check (vs. bluff?) as a minor action could be allowed to reveal what it's looking at, if not the precise trigger.
The single melee basic attack just isn't this big a problem, in most cases the monster is better off just risking it. Why would you ever risk both an opportunity attack and a combat challenge attack? If you wish to make a ranged attack, then shift away first, then fire your weapon. You might get hit, but you will not be stopped, and you can then use your ranged power safely.
Technically, if you are next to a fighter and marked, and you use a ranged power to attack an ally of the fighter, you will provoke both the OA and the combat challenge attack. Rather than forbidding the fighter from (ab)using that, I'd prefer not running my critter with such bad tactics - why would you use your ranged power next to a fighter if you can just shift away? If you can't shift away, then the critter might (deservedly) suffer for the disadvantage.