I do not believe that Voln symbols should be able to be dispelled by bandit's yet I had this happen tonight, I was advised to post it on the forums
Had a bandit dispel me and with it he took Symbol of Courage

A half-elven marauder pulls out a tiny shard of jet black crystal and hurls it at you!
The crystal shatters against your body and releases a pulse of anti-magic!
You no longer bristle with energy.
You feel the extra courage wane.
The glowing specks of energy surrounding you suddenly shoot off in all directions, then quickly fade away.
The powerful look leaves you.

To the best of my knowledge voln symbols should not be dispellable

You also see the Shilarra disk etched with the image of a Vathor