honestly, the solution here is simple.

walk away quietly. no hysterics, no "oh my god I hate you and now I'm going to fucking play Dashboard Confessional until my ears bleed," no one-last-time-anger sex, nothing. zip. zilch. nada. just return anything that's hers (all of it) and get what's yours back (hint: none of it) and be done with it. the surest way to really win any sort of conflict is in forgiveness and the absolute absence of memory. when she tosses out the inevitable questions about working it out, or trying again, or ANYTHING for that matter, just use the best response ever:

"this relationship no longer serves a purpose." that's it. nothing more. no name-calling, no mentions of parental-coital retribution, nothing else. "this relationship no longer serves a purpose." if you want to add some venom to it, then add in a sincere "I hope we each find what we're looking for." after that, delete her. any sort of link you have to her, eradiate it. Myspace, facebook, livejournal, hardslutswhotakeitinthebutt.com, whatever... gone.

personal experience: I did this and it worked. I (stupidly) dated a girl who dumped me TWICE for ugly lesbians (long story, I'm a retard, let's move on) and I, being stupid, took her back. Third time? That was it. Totally excused myself from her life. Removed all references to her, removed all communications to her, changed my phone number (not needed, but I *was* feeling rather dramatic at the moment), and just wandered right along with my life. Best. Decision. Ever.

she messaged me on myspace the other month.. told me how she missed me and hopes my new girlfriend is "truly deserving" of me. surprised me because I figured she wasn't keeping up.. guess she was.

I win.