Depends. Power attacks, and many abilities, do indeed take some time to 'cast', and thus can be interrupted, either by standing next to them and pressing both left and right mouse buttons (same as for breaking CC), or by using one of the many weapon-based interrupt abilities, such as:
Dual Wield:
[Active] Hidden Blade – Throws a dagger that deals physical damage and interrupts the target, snaring them for a moderate duration.
Bow:
[Active] Poison Arrow – Deals immediate damage to the target and poisons them, interrupting enemy spells, stunning the caster and putting them off balance.
Destruction Staff:
[Active] Force Shock – Deals damage and interrupts enemy casting. Interrupted enemies are stunned.
Also, Templar has a couple of interesting ones:
[Active] Focused Charge - Charge to an enemy, interrupting them and dealing Magic damage.
[Active] Rune Focus - Creates an area of self protection, gaining immunity to interrupt effects and taking less damage while the effect lasts.