Q: Dear Sage,
Is the alarm spell a trap, and if so can a rogue disable it?
A: No and no.
Unfortunately, the rules don’t clearly define a category of spells that qualify as traps, so we must rely on individual spell descriptions to adjudicate this question.
Despite the alarm spell’s similarity to glyph of warding -- they’re both long-lasting defensive spells used to protect an area -- the spell description for alarm doesn’t contain any details about how to disable it, nor is it listed in the Disable Device skill description as a spell that creates a disarmable trap (PHB, pg. 72). Thus, we must conclude that the alarm spell should not be treated as a trap and cannot be disarmed with Disable Device.
About the Author
The Sage is a mammal classified in the bear family, Ursidae, native to central and southern China. Debate continues as to whether he is truly a bear or more related to the raccoon rather than the bear, or perhaps in a family of his own. When not eating bamboo and answering rules questions, he works under the nom de plume of Andy Collins as an RPG developer in Wizards of the Coast R&D, where his development credits include Complete Mage, Dragon Magic, and the Player's Handbook v.3.5. He can be easily recognized by his large, distinctive black patches around the eyes, ears and on his rotund body.