Can a stroke or closed head injury directly affect your personality?

Yes: There are areas in brain that controls emotions, impulse, social appropriateness, temper - so people can have rage, impulse control issues, disinhibition, outbursts, difficulty coping.

Absolutely: One would think that in this modern age, people would have this basic understanding that the brain is the seat of the personality.

Yes: Particularly if there was damage to the frontal lobes.