I always had trouble with giving off the wrong vibe, I just never learnt the art of *not interested*.
Mainly because when I am out, I am usually drinking and therefor go into hyper super friendly mode and I'll talk/flirt with anybody and everybody....so guys usually take that as "i'm in!!"
I always get the guilts if they try and buy me drinks and I know I'm not interested.
Sometimes if my sister has been annoying me, I'll give out her phone number