This bitch needs to stay away from me.
Did she do something to make you not like her?
She's someone I met at a therapy group. She's seems so fucked up that I can't stand being around her.
Being annoyed with her is fine but just telling her to go away without a reason is pretty rude. This was a therapy group so she might just be reaching out because she things you would be a good friend and can help her. If you're going to tell her to go away at least tell her why so she can work on it.
I hate her, that's all I need to tell her to piss her off.
It is fine to want space, but how you are doing it is rude. You should tell her calmly that she shouldn't contact you any longer.