Unarmor
Lock Joints (only works on complete armors)
Heat Armor
Cool Armor
etc
In a kind of way those spells do exist:
Sorceror Base
Solid Destruction contains: Shatter, Umetal, Solid Destruction True.
Closed Mentalist
Solid Manipulation: Bow Break, Blade Break.
I would allow to apply Bow Break against some of the armor straps.
Maybe the caster need experience (Spell Mastery) to know how to rip down the armor.
But imagine someone "shatter"s your armor piece by piece.
"Heat Solid" exsits on different lists.
OK, a new anti armor list will combine the different spells.