Can I stop someone from visiting my Dad of whom I am guardian?

Yes, absolutely and you can get an Order of Protection if the visitor is acting in a predatory manner, asking for money or upsetting your ward. Read your court order appointing you. (I am a Guardian in NY)

Yes. If you think someone will upset the apple cart, you can keep the person away. A guardian is very much like being a parent. Just as a parent can forbid their children to see certain people or do certain things that may be harmful, a guardian can keep people from their "charge." The guardianship should be used with wisdom, though, so that actions are for the good of your father and not a personal vendetta.

I agree with Jessie and Pam. I also think one doesn't have to be a guardian to keep troublemakers away, the main caregiver can keep troublemakers away as well.