Once you declare/start an action you don't get to change the action. I am not saying you must run all the way across the battlefield, but stopping and then choosing a new action would defeat the purpose of the other person's outsmarting the charger.
Think tripping through a beehive into a bucket of tar, rolling downhill, running into a flour barrel, breaking through the blacksmith's shop and ending up in the forge.
For less cartoon in your game, charge is broken into two parts - the movement and the attack. They're treated separately in the rules, probably for reasons like this. The move part would occur but the attack would hit nothing. Essentially, kudos to the spellcaster for thinking ahead and preparing.
On readied actions:
If the triggered action is part of another character's activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.
On charging:
Movement During a Charge:
You must move before your attack, not after. You must move at least 10 feet (2 squares) and may move up to double your speed directly toward the designated opponent. If you move a distance equal to your speed or less, you can also draw a weapon during a charge attack if your base attack bonus is at least +1.
Attacking During a Charge:
After moving, you may make a single melee attack. You get a +2 bonus on the attack roll and take a –2 penalty to your AC until the start of your next turn.
