how should a DM handle this if a player does attempt an attack?
"The Bard attempts to destroy the iron bars of your prison, casting Vicious Mockery. The iron bars show no sign of being shamed, or in fact any other emotional response, and remain unflinchingly solid."
There would be no problem in the game as such if that were not the case in the rules system (a few hard-to-imagine Martial powers aside, such as a Rogue "outwitting" a door), but Psychic damage and powers that target Will have other benefits - generally these are good choices against a wider variety of monsters, compared to powers that e.g. target Fortitude or that do Lightning damage. So it isn't unfair on those players based on Will attacks, that their characters' powers cannot be used to damage objects.