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 even 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.

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.