I'm wondering if there's a typo in the dispel magic description:
Target or Area: One spellcaster, creature, or object; or 30-ft.-radius burst
If the bolded text said, "spell," then I'd agree. And later, as you point out, under targeted dispel, it says:
Targeted Dispel: One object, creature, or spell is the target of the spell.
The nasty thing about dispelling it with dispel magic, if you wanna be a rules-lawyer about it, is that they wouldn't get a reflex save to jump clear (as they would if you used transmute mud to rock). I'd probably give them the same reflex save, though, because to do otherwise just feels a wee bit too metagamy.
It does occur to me that a good tactic would be to ready an action to muddify the ceiling above an enemy spellcaster once she starts to cast her spells. Bam! 8d6 points of damage -- concentrate check THAT, baby! That puts me right before her in the initiative order.
Next round, I do an AoE dispel magic centered on her. Automatically dispel my own spell, and take one buff off everyone within 30' of her.
Heh heh heh. Gonna have to try that.
Daniel