Well, any form of extra-planar travel would work.
Any movement (t-port, d-door) that can put the attacker right next to the defender defeats alarm (though it still goes off). Heck ranged attacks work well too (combine with a cow catapult for extra fun).
Dispel, Greater Dispel, and Antimagic Fields all have obvious uses.
Finally, there's always the "decoy" tactic; the spell goes down after it's been activated once, so just send in a blind kobold until the wizard can't cast Alarm that day, *then* attack. For really good results repeat over many nights.