A slightly harsh and dismissive way of saying this is : 'She's nothing to do with me'.

Yup, I'm OK with that (though it's friend, not firend). the next bit is: If I say IT like this is it OK? (you forgot the it). the "She's nothing of mine" is wrong. You could say, "I don't even know her". Or "She's someone I know, but she's not really a friend." Or "I've never seen her before." It all depends on what the truth is. Should I write out the restraining order now? You might need to look that one up!!!