In articles 4 and 5 of ST I-II, 6, St. Thomas addresses how violence can affect the will and the voluntariness of acts.
Violence in this context means an outside force acting against an interior inclination. A violent shove, for example, might act against your interior inclination to walk down the street.
In article 4, St. Thomas says that violence cannot be done directly to the will. He describes the act of the will as twofold: the "immediate act" of the will that we might call "to wish"; and an act of the will commanded by the act of wishing, which in turn commands a person's other powers to fulfill that wish (by, for example, walking somewhere or saying something).
While violence can certainly prevent a person from walking somewhere or saying something, it can't prevent a person who wishes to do so from wishing to do so.
(Violence can cause a person to wish something he wouldn't have wished if not for the violence, but that's not the same as preventing him from wishing something he is wishing.)
The fact that violence can't be done directly to the will doesn't, however, mean that violence can't cause involuntariness, as St. Thomas shows in article 5. The acts of the will are all voluntary, so whatever acts against the will is called involuntary. (Similarly, whatever acts against the natural appetite is called unnatural.) If I am forced by violence to sit where I do not wish to sit, then my sitting is involuntary (and therefore not an act that is either good or evil).