Open your weapons/mp/smoke_grenade_american_mp (or whatever your filename is) and search for "damage" in Notepad++ and replace it all with 0:
meleeDamage doesn't need to be 0, but shouldn't matterCode:damage\0\meleeDamage\50\explosionRadius\0\explosionInnerDamage\0\explosionOuterDamage\0\