If you are knocked down, any spell that you're casting/skill that you're using will be interrupted- even if you're using glyph of concentration or one of the 2 mesmer mantras that prevents interruption. Knockdowns are treated differently from interrupts, and a skill that prevents one won't prevent the other. Thus, Balanced Stance will prevent you from being knocked down, but any skill that interrupts your spell/action will still interrupt.
If you use both Glyph of Concentration and Balanced Stance, your spell won't be able to be "interrupted" by any means- though things like Blackout can be used to prevent you from being able to cast the spell(if your blacked out while casting a spell, the spell gets cancelled), and they can always try to kill you if they really don't want the spell to go off.