My flatmate's parents are planning to visit. I really wish they wouldn't.
Why don't you want them to visit? They are your flatmate's parents after all.
I hate them for being so hard to talk with. They always blame me when he has other problems
That sounds like a pain, but your flatmate is allowed to have their parents over. You don't control them, and preventing their family from visiting would probably only cause more problems.
That's not fair. I just want to be alone and not have people visiting.
If you want control over who comes and goes so you can choose to be alone, you shouldn't live with a flat mate. It sounds unpleasant to have them there, but you can't control it. Maybe you can take refuge in your room or go out and do activities away from home when they come.