Changing Enemies Damage Type

[Vaz]

For example, you have Damage Reduction/Piercing; is there any way (effectively) to change an enemies damage type to bludgeon?

A Spell, power, tactic etc?

The easiest way I can think of is by disarming the target, but are their any others? Such as a spell/power which changes a targets weapon to a particular damage type involuntarily?

[HalfQuart]

Well, you could cast Earth Hammer (RoS, Cleric 5/Paladin 3) on the enemy's weapon. It would require a touch attack and a Will save. That's all I can come up with.

[Feralventas]

If you're undead, you can cast the brilliant blade spell to make your enemy's weapon a Brilliant Energy one so it doesn't hit you.