it's "you call the guy ' , either directly or asking mamasan to.
It's extremely rare that the guy approaches you on go-go bars and I'd say only endemic to The One(Tawan) and likey Atlas as many their boys migrated there. And even there boy will not sit with you unless invited. Mamassans , sensing newbie are another story . In either case " no, thank you " suffices for an answer .
Of course different situation is if you show up in the bar where you offed the same boy few times. As soon as he sees you he may treat you as his property and invite himself to sit beside but even that is not general occurrence.
In general they take ' no ' for an answer without much fuss so don't feel terrorized , just go and have fun.
No need to tip anybody who did not sit with you unless you want to- this is how I signalize to a boy my interest in him on future visit.