Is it permissible for the husband to use black kohl to cover baldness, and does this kohl prevent ablution (wudu'), and what should the wife do to prevent him from doing that?

There is no harm in applying kohl to a bald spot, and this is not considered dyeing with black, which is prohibited. However, it is preferable for men to avoid it unless there is a disfigurement. As for its effect on the validity of Wudu' and Ghusl (definition), if it does not have a substance that prevents water from reaching the skin, then it does not affect them. Otherwise, it must be removed.