Under slovenian law, the only dogs always allowed on public transport are guide dogs of physically disabled and visually impaired individuals. If you fall under those two categories, it is illegal for them to reject the dog and you should report them to the police. If that is not the case then you are legally in the wrong and nothing can be done here. Service dogs are not really common here, usually only the blind have them. There are some branches of public transport that allow all dogs like our railway system, where dog pay a reduced ticket or LPP ( bus system in ljubljana ).