give her an ultimatum.
tell her you are fooling around (not dating) with another girl and you dont want to stop.
and jokingly say that if shes cool with sharing (fooling around, not dating) you with her then she can have at ya.
if shes all for it. bam you got a girl on the side without the drama.
if she hates the idea, well you just got her to leave you alone :D