To vocalize "don't call back" makes it real and depending on how you said it can give her closure. You tell a girl not to call you anymore, there's a strong possibility that she won't call you anymore because she knows why you're telling her. Although having sex with her will give a better chance that she will call you back because sex cements emotion. Depending on the situation, telling her not to call back will also in her mind justify her actions.

Cutting contact with her is better because there is no closure and she may not be sure of what she did wrong in your eyes. It will more than likely cause her to find out what went wrong which causes more of an emotional investment in you.