Actually, Runed braces does stop magic attacks (not just spells). I just tested, made it 1.00 (100% resistance) and got a priest to attack, and it did no damage.
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EDIT: ignore what i said. It actually doesn't block magic attacks (sorry) I was testing something else. But i did find out a way to block magic attacks.
Go Advanced -> Gameplay constants -> Gameplay - Magic immune resists damage and tick it off( set to false). Then make a custom Magic Immune ability and find Data - Magic Damage Factor and set it to 100 for 100% resistance to magic attacks. 10 is 10% resistance, 40 is 40% etc. But they will be immune to spells, so that sucks. :/
EDIT: IGNORE EVERYTHING, LordDz had your answer the whole time, what a wasted effort to get some rep lol