Great question! Text review:
Would Retribution acing effect be stopped by Shield of Faith?
Shootout Miracle X, Boot: Choose an opposing dude. X is that dude's value. If that dude's controller reveals a cheating hand this round and you reveal a legal hand, ace that dude. This acing effect cannot be prevented.
Shield of Faith
Shootout Miracle 7, Boot: Dudes cannot be aced or discarded during this shootout, except as a casualty for losing the round or due to their owner's card abilities or traits. Reduce your casualties this round by 1.
Ruling regarding rare circumstances of two cannot clauses:
"In rare circumstances, you might encounter two “cannot” clauses that would both override each
other. In these cases, the Winner decides the order of preference on these conflicting clauses."
We are going to reference the ruling on Shelby Hunt v. Heartseeker:
Shootout: If a dude in the opposing posse has higher bullets than Shelby, Shelby gains +1 bullets, is a stud, and cannot be affected by other players' Shootout abilities.
Shootout Hex 5, Boot: Reduce your opponent's casualties by three this round. If you win this round, ace the opposing shooter. This acing effect cannot be prevented (even if the shooter is Harrowed).
There are two 'cannot' effects in this situation (Shelby cannot be affected, Heartseeker's acing effect cannot be prevented), so we're applying the 'specific beats general' principle.
Heartseeker's very specific 'this acing effect cannot be prevented' overrides Shelby's more general 'cannot be affected by other player's Shootout abilities.
Retribution overrides Shield of Faith because it is specific.
We understand in reviewing older threads regarding Heartseeker that there were some unresolved issues and are looking to make sure these are resolved and rulebook updated to be consistent with the specifics of Prevention as opposed to the cannot v. cannot cards.