What is the Islamic ruling on forcing an adult female to marry against her will?

Islamic law accords the right of guardianship to some individuals to protect the interests of their charges and ward off harm. This right allows guardians to exercise their will over their charges. Islamic law considers mental incapacity and legal incompetence two reasons for establishing guardianship; the existence of one of them constitutes sufficient grounds for establishing guardianship over another person.
Scholars mentioned [different] types of guardianship: guardianship over the general affairs of a person and guardianship for the purpose of marriage.

Guardianship for the purposes of marriage

This type of guardianship is consistent with the aforementioned definition since it involves executing the guardian's will over his charge. Ibn Abdin stated in Dar Al-Mukhtar that "guardianship is executing one's will over another."

The Hanafis

Hanafi scholars maintain that guardianship for the purpose of marriage cannot be established except over minors because of their age; therefore this kind of guardianship cannot — under any circumstances — be exercised over an adult female. According to them, it is recommended and preferable for an adult female to have a guardian. The order [as to who has the right to be her guardian] is the same as that outlined by the laws of inheritance. Al-Zayla'i mentioned in Tabyin Al-Haqa`iq: "Guardianship in marriage is of two types: The first is the guardianship over a sane adult female whether or not she is a virgin, and this is recommended and preferable. The second is the guardianship over a female minor with the authority to compel her to marry without her consent whether or not she is a virgin." He [also] mentioned in Kinz Al-Haqa`iq: "A guardian has the right to marry off a minor, male or female; the guardian is a universal guardian occupying the same order as that in inheritance."

The Malikis and Shafi'is

The Malikis and Shafi'is maintain that guardianship, with the authority to compel a female to marry without her consent, is exercised over both virgins of any age. While the Malikis maintain that only the father is entitled to guardianship, the Shafi'is maintain that both the father and grandfather are entitled to it. Citing Malik, Al-Baji, the great Maliki scholar, mentioned in Al-Muntaqa that only the father can compel a virgin to marry, not her grandfather or anyone else.

Imam Al-Suyuti, the Shafi'i scholar, mentioned in Al-Ashbah wal Nadha`ir: "Certain regulations have been outlined concerning the father and paternal grandfather, among which is guardianship for the purpose of marrying off a son or daughter without their consent."

The Shafi'i scholar, Al-Khatib, mentioned in Mughni Al-Muhtaj: "A father has the right to marry off a young or old virgin without her consent [although] it is better to seek her permission; he does not have the right to marry off a non-virgin except with her consent. If a female is prepubescent, then she is not married to another until she attains puberty. In the absence of the father, the grandfather takes his place."

Impermissibility of marrying one's daughter to an insane man
Imam Al-Shafi'i maintained that it is impermissible for a man to marry his daughter to an insane man. In his famous work on Islamic law, Al-Umm, he wrote: "It is unlawful for a father to marry his daughter to someone suffering from leprosy, vitiligo, insanity, or someone whose testicles have been severed, even though he may be a suitable match. The reason is that if she were an adult, she would have had the right to choose if she had known of these defects in advance.”

The ruling

It is impermissible to marry off an 18-year old female without her consent even if it were to a suitable match. In addition, marriage to a person who is considered insane such as to a permanently retarded person is another reason for the objection to the validity of this marriage. This is because marrying an adult female to such a person is impermissible except with her consent.

God the Almighty knows best