So you've outmatched the wizard by removing the vocal aspect of his spellcasting... very clever... (scribbles that down for future reference).
He throws a sharp glare at the party, unable to separate his lips, and flees. "Next time!" he attempts to yell, but nothing more than a few "Mm-mm-hmmm's" escape him.
Well, as the DM, you have ultimate ruling on all of this. The bad guy has been thwarted this time, but there's nothing stopping him from freeing himself now that he's away from danger. Either through a comical montage of his attempting to peel his lips apart by hand, or crowbar, or some crazy mechanical device he whipped up just for this occasion; or finding some magical spell or potion that can dissolve or dispel the gluey seal.
But when it comes to the rules, there are also a few saving graces that can just allow him to free himself. Firstly, the duration:
Duration: 1 minute/level (D)
Even when cast at level 20, this means the spell only lasts 20 minutes. Eventually, the wizards lips will become unsealed. Additionally, this can be dismissed at will by the caster (though, likely would not be).
Secondly, technically the target is not the wizard, per se, but more so something that the wizard is using, so the "item target" section of the spell description does allow for the target to make strength checks and combat maneuver checks to attempt to overpower the spell on each of their turns. However, they would need to do this each time they opened and closed their mouth (emphasis mine):
it cannot be separated from the other item unless the creature succeeds at a combat maneuver check or a Strength check as a move action to free it (DC = the spell’s DC). A creature must succeed at saving throw each time it attempts to use the sticky item; otherwise, the item becomes stuck to the creature.
This would still make casting spells with Verbal components incredibly difficult (and I'm not entirely sure what kind of combat manoeuvre can be used - maybe using a knife to cut through the seal... carefully). That said, this doesn't mean he would be incapable of casting other spells - any that don't require the Verbal component.