It takes a little while to get use to but you have to learn to say 'NO' as usually it's the more desperate and ugly chicas that try to appoach you even being so bold has trying to sit with you, and if they fail they will try to hit you up for at least a dollar, so it's very important to learn to say no; the top tier chicas won't approarch you.
Yes, if you want to be nice "Quisas mas tarde" works, two of my favorites are simply ' No tengo dinero' [ I have no money} If I want to be an ASSHOLE' or if I think they are a little rude to me, I'll tell them I'm a Maricon, Joto, or just tell them you're gay in english. I have also told them I'm looking for a 'Travesti' [tranny] If they don't believe you and don't go away, tell them OK, but sex 'sin-comdom' [ without a condom] and you want ANAL sex too, if they say ok, you might want to re-consider and take them UPSTAIRS.