Just not effective. I'd rather just sling a Fireball or Magic Missile and stop him casting that way.
The reasons why kenders suck is that they are horribly gimped in just about every possible endeavour except being thieves. Unfortunately, they are not even the best at it, and to top it off, thieves are only absolutely required in one instance in an optional dungeon in the postgame of one of the games. You are always better off playing other races that has any x/thief multiclass.
Does that make your playthrough less enjoyable? Completely irrelevant. The premise you proposed was why kenders suck, not whether you have a blast playing them.
Two points here:
1) Whether I have a blast playing them or not is relevant, as games are meant to be enjoyed. Otherwise, why bother?
2) It's clear you think having an optimized party is about damage output, which it isn't. If I have, for example, a Kender Cleric/Mage, you would likely find that sub-optimal. That's merely an opinion and not fact, though, since there are multiple ways a character can contribute to victory besides dealing damage. A Kender Cleric/Mage can utilize their taunt ability to enrage enemy spellcasters, cast several healing spells on party members, use Dispel Magic to help characters who have been charmed or held, cast Stinking Cloud to foil the enemy ability to move or to keep spellcasters from casting, use Charm on an enemy to make them a temporary ally, etc. None of this makes that Kender suck, and in fact, may turn the tide of the battle in a much more effective way than dealing damage; because your Fireball may not bring a large enemy down, but my Stinking Cloud might.
This is the same argument I have made all along when people have said you shouldn't play multiclassed demi-humans in these games, to which my answer is that it is complete bullshit. I have beat all of these games with the exact same party (except in POR, which doesn't allow certain classes), and they always include two multiclass characters who do just fine, even in the high level games.