That first tour can be tough! Even if you've read the posts in places like this and talked to friends about it, there is no way that you can understand what it is really like until you have experienced it!
As for telling a BG to clear off. When she asks for a drink, say No. Order your own drink at the bar from the bartender, don't let her take the order for you. If she asks you if you want to play a game, say no, thanks. Be polite and smile the whole time you are saying no to her.
If you would like to meet her friend, tell her you just want to be "phichai"(older brother), which means I just want to be friends with you. Call her nongsao(younger sister). Then ask her to introduce you to her friend! By telling her you want to be phichai, you are showing goodwill, but making it clear that you don't want to go with her(friends don't f*ck friends, even in LOS!). Other ways to let her know you are interested in her friend, are to ask her for the friend's name, or to ask her if her friend has a boyfriend yet.
I think that usually if you are warm and friendly, but very clear about your intentions, that they will rather give their friend a shot at you than see you leave alone.
If you just want to enjoy your drink alone, then smile, be friendly, and say, "Dtong gan khon diow."(I want to be alone).
Phichai sounds like pea-chai (chai rhymes with eye). Dtonggan khon diow sounds like "Dtong-gun cone dee-owe."
JayBee
P.S. It is funny how words like older brother, and phrases like "I want to be alone" which would probably be very awkward to say in English, are very easy to say in Thai, and make perfect sense to the girl you are talking to. They get your point across right away in terms that they can easily understand.