[Jack Simth]

That would be even simpler to divert - send off a ball of magical energy/golem/whatever that travels at the same speed the dust does (perhaps watches the dust to do this) - the dust follows the energy ball/golem/whatever which heads away from the caster.

What you really want to do is coat a dart/arrow/whatever with it. The caster can't divert it magically because the dust absorbs any magic sent its way. When it hits, the dust gets into the caster's system, and prevents casting until the dust is burned out; it can't even be cleaned off as it has gotten into the caster's blood.

The defense would be to use a method of moving things that doesn't touch the object moved with any magic (like the way Jack moved the ring), or block it with some physical object (like a shield or golem), or dodge by some method. If a blocking method is chosen, and the physical object is being manipulated magically, the spell that moves it needs to be physically protected from the dart, or else it will need to be recast.

It could also be formed into blocks and used to make magic resistant walls to imprison a mage, or hide from one.