Is it obligatory to impose the hijab on a mature girl even if she refuses to wear it, and will forcing her to wear it diminish her reward?

A girl must wear the hijab once she reaches puberty, and her guardian is obligated to enforce it upon her. If she is coerced into wearing it, she will not be rewarded because she did not intend it as an act of drawing closer to Allah. Acts of drawing closer [to Allah] are only rewarded through intention, as the Prophet, peace and blessings be upon him, said: "Actions are but by intentions, and every person shall have but what he intended."