[josh_S]

I know this is old, but I think you should have just anonymously let the beta know she was effin you still. Let him dump her, which he would do, before it got serious. She would run back to you and you would act completely disinterested and make her beg and grovel for any type of attention, slowly rewarding her for sex until she had learned her place... but never let her win it back... let her know about one of the other plates your spinning (or maybe just always mention the other girl) so that she never feels comfortable or like she got back to where she was with you before... then she won't do it again for fear of losing what she did get back.