What you are asking for is impossible. Know that she is going to be hurt no matter how you break up with her. Just try to do it in the least asshole-ish way possible.
That poor girl! Why would you be with someone for so long and remind her throughout all of that time that you won't be together forever? And to top it off you laugh about making it so that she cannot contact you. I bet the truth is is that neither of you were joking.
THEKingofLibra, as other people have also stated, is right.
Just tell her the truth, break up with her, and let her find someone who can treat her better. It's got nothing to do with star signs.
Agreed