Whenever you cast an instant or sorcery spell, target creature an opponent controls can't block this turn.
Resolving Smelt-Ward Minotaur’s triggered ability after a creature has blocked won’t remove the blocking creature from combat or cause the creature it blocked to become unblocked.
Smelt-Ward Minotaur’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.