Why is my 4-year-old Black Lab mix biting all of a sudden?

I've had her since she was 5 weeks old. She's always been a good dog...loving, affectionate and playful. But I've noticed a change in her behavior in the last week or 2--she won't let ANYONE outside of family pet her and she growls and snaps if they try! We do reprimand her for this. What's worse is she is doing it to people that she sees regularly and today I found that she bit my younger sister (she's 23) unprovoked. I'm worried that there's something wrong. She's current on all her shots, so I know that's not it. I know that there's been an old man with Alzheimer's disease in my area that's been mean to her...he growls at her, yells at her, and runs at the fence (her reaction--she barks and howls and runs around in circles). We've asked him several times to stop and even talked to his family, but he still does it. I'm afraid she'll bite him and then I'll lose my dog! Can someone please help?! I don't want something bad to happen to her!!