Bitch Shield and say "no, she has a bf too, sry" trust me i've had that before even when i didnt ask.
i love to say this:
her: "i have a bf"
me: "oh really? i heard bf's are good for opening doors and pulling chairs away. can he also cook for us? "
i usually get the "haha nope" with the look -you're a smartass-
but hey, better than just walk away.
i also use
"oh wait a minute, you thought i'm hittin on you? i just want to make friends so you can buy me lunch sometime, your bf can come too.. i guess.." i barely use it but sometime when i feel like in the dick mood i will