Mental attacks, status attacks and damage attacks (melee or packet delivery) are all valid responses to Taunt.
Now, if the target is in Hero’s Stand and they have no other attacks available to them outside of a melee boffer strike, the Hero’s Stander should attempt to close the distance, even though it would cause them to drop to zero Health Levels.
This is not that different from other mentals that can break Hero’s Stand, and Taunt is a spell that is, at its core, magically forcing the target to want to hit the caster above all else, not sit there and wait.
It might be worth noting that you can still resist mentals with willpower while Hero’s Standing.