Depends, I have had two pcycho x's.
The one of them were so bonker that I completely blocked her, but that's not the best sollution.
Send her a message "no", but write something in it. Just say the usuall stuff that you are sorry things turnes out this way, you never meant to hurt her, you did not lie to her but it turns out your feelings for her is not what you wanted them to be. After all - feelings are feelings, you canot controll them,
If she does not accept that answer, then reply "sorry, but that's the way is has to be, it's for the better for both of us". If she still is bonker, then you start ignoring her. Don't answer SMS'es and don't answer when she calls.
Just my advice
Edit: And lol at the love guru statement
Most of us, at least me, is a misserable **** who canot even keep a GF for a longer periode, so maybe ignore everything I says