Pummel the target, interrupting spellcasting and preventing any spell in that school from being cast for 4 sec.
The first, best solution to anything." 
Pummel is a single target instant warrior attack and debuff, that costs 10 Rage. It does no damage, but if applied during a target's 'Casting' charge-up, will make the spell fail, and prevent casting of any spell in the same school for 4 seconds. It has a 10 second cooldown.
Very similar to the Shield Bash ability, and the same flair for timing is required to make this work. Although Shield Bash does a better job, with an extra 2s lockdown, and enhanceable through talents, having this on the Berserk Stance toolbar affords extra flexibility; letting you do something about the Evil Wizard without having to lose rage through a Stance Swap, and possible shield equip.
Most outdoor PvP raiding is too hectic for this to be that useful, but in a one-on-one PvP duel, this is definitely a match-winner for the otherwise magic-vulnerable warrior.