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.