This card exists. Dispel is functionally identical:
Dispel: U Instant - Counter target instant spell.
With the exception of the rare flash creature, dispel counters anything an opponent could play on your turn. It is actually a little better than Spell Ward because it can also counter instants on your opponent's turn.
It isn't broken, although it does see some play. I think duress and thoughtseize are better in combo.
Well there's the answer. Thank you, Eastman.