To steal mats from other's Disenchant rolls in a dungeon, you need to:
a) be a rogue or blacksmith
b) be an enchanter as well
c) have a locked chest containing a green in your dungeon.
How to:
Shift click while picking the lock on the chest. Or turn off autoloot. Whatever. When people roll to disenchant, the mats get stuck in the chest. Since you're looting the chest, you can just loot the mats that should have gone to the other person. Congrats, you're a bad person now.