Thanks, kinda
Thanks for all the replies, at least some of them, the helpful ones, I mean.
Background: Player X's wizard looks like a wizard. Robes. No armor. No obvious weapons in hand. Maybe a dagger on the belt. No disguise spells to look like a Paladin or whatever.
In a world where magic is extremely commonplace, a wizard is more dangerous than anything short of a raging barbarian (within charge range).
If you are a fighter with 2 of your buddies, and come across hated enemies in a cave somewhere, and one of the 4 hated enemies is obviously a wizard -- that's top priority. More scary than the archer type, the cleric type, the fighter type. One spell (fireball, whatever) can kill you -- period. Dead. Done. It takes the fighter at least 3 rounds to accomplish that (ok 2 or 1 with a critical).
Point is, "readying" an action to disrupt the spellcaster is a no-brainer obvious tactic. Int=10 gets you there easily. If you disrupt the spell, you a) don't die in a blast of fire, b) hurt him, c) maybe kill him.
What's not to like?
On the other hand, plunking a few arrows into the cleric or fighter will probably a) miss, or b) hit and do some damage, very unlikely to c) kill the heavily armored character.
??
Result? One of the fighters gets out his bow and readies an interrupt action; the other two fighters advance into melee.
So yes, in answer to some questions, intelligent (i.e. 10+ int) opponents in my world will very often ready an action to interrupt the spellcaster. Or try to do other thing like grapple, throw sticky bags of goo, nuke 'em with alchemist fire, etc.
It's not very common that enemies know about the PC's detailed tactics. That's very rare. In fact, I'm not sure how that would happen, unless an enemy escaped a battle and blabbed (and had spellcraft and knew what was going on). Could happen. But...rare.
In D&D spellcasting is very powerful. Very powerful. My main objection is that this tactic allows a PC, from low to high level, to get around readied actions that stop an important spell from going off.
Somebody said well what's the big deal? just grab a metamagic rod of quickening and you can get off spells anyway -- ?? Isn't that a problem with the rod, for goodness sake? And who wants to spend all that money when this is so much cheaper and easier? Hello?
I think even a Wizard11 should be worried about getting a spell off when surrounded by archers -- not all of them low-level grunts for goodness sake. This tactic nullified a high-level archer opponent's ability to dish out 25 points of damage per arrow and stop the spellcaster cold -- or at least have a fighting chance to do so. It hinders the balance greatly I feel.
I'm certainly not "out to get" the PC. He's used his tactic to good advantage, probably once every other combat. I did take away the cantrip, true. That's my perogative; I didn't forsee how powerful a free action spell could be. So sue me (sosumi).
I am "out to balance" things somewhat. I'll take the good suggestions and Idea's I've gleaned so far, and throw out the bad. Unfortunately, the tactic does seem to be legal... at least according to the latest point/counterpoint. I think.
[I would respectifully suggest that, by some of the posts written here, that it is likely that other campaigns feature intelligent creatures that don't use "readied" actions nearly enough -- but I can't be sure of this. It's not "strange" or "unusual" for an intelligent creature to take pains to save its skin. That seems pretty obvious to me. It shouldn't be seen as a repetitive or boring tactic. Is swinging a greatsword, time and time again, boring and repetitive? NC from me.]
BTW Player X has a good sense of humor about the thing and knows the cantrip he designed was a way to "screw" the system; he took it with good grace when I removed it. But he does still use Feather Fall without shame. Though still with a bit of a smile. In my direction.