If Venser's triggered ability targets a spell cast with flashback, that spell will be exiled instead of returning to its owner's hand.
If a spell is returned to its owner's hand, it's removed from the stack and thus will not resolve. The spell isn't countered; it just no longer exists. This works against a spell that can't be countered.
If a copy of a spell is returned to its owner's hand, it's moved there, then it will cease to exist as a state-based action. It can't be recast.
Standard: Ej legal
Pioneer: Ej legal
EDH / Commander: Restricted
Pauper: Ej legal
Brawl: Ej legal
Historic: Ej legal
Premodern: Ej legal
93/94: Ej legal
Venser, Shaper Savant är Legal i dessa block: Time Spiral