YOu can't dispel a wall of force, either. You can only destroy it. It's one of the things that stay around even in an AM shell.
And there was no upper limit to AC in 2e, only 'highest published'. Holy Sardior, someone remembered the gem dragons?!
We got a drow fighter/cleric in the Vault of the Drow down to -20 AC with spells without too much trouble. Shocked my brother when his 15th level dwarf with the dwarven thrower kept missing her!
Actual there was a limit in 2nd edition for PC on page 73 of (black DMG) under the definition of AC. the limit for pc was -10, this is also reflected in every 2nd edition video game that was ever created. While you could find enough gear to get to a -20. The games did not allow per those rules. There were only a few exception and I think almost all of them where All Dragons ages 11+ with the best AC possible in game was age 12 gold dragon, Shadow Dragon and Amethysts Dragons all had -12 AC, Red had -11 at age 12. I want to say there was only one non-dragon monster that had a -12, that was not in a deity or demi god, but I can't remember what it was.