Damaging (or removing, without tearing other parts) the helmet has been done plenty of times in the past with the physical force alone, despite Black Tom working on it and adding little upgrades of his own to make the task as difficult as possible. Eventually adding the skullcap (made out of the same material) underneath, too.
Sure, it shouldn't be possible, given how the armor is a part of Juggernaut's power and the forcefield should engulf the whole thing & protect it.
Nonetheless, it was always easier to break the armor than the man himself, the battle against Exemplars is a nice example of that:
BFR and the mental assault have remained the only surefire ways to take him down.
Unless he is facing an opponent relying on psionic attacks, there is not much use for the armor, as seen when Juggs in his civies beat the living shit out of Professor Hulk or humiliated Colossus in a bar.
Both of them did affect the helmet on a few occasions, but being able to do that did them no good in the actual physical confrontation. In order to actually benefit from the helmet removal, they need others - telepaths, specifically - to step in.