Pls dnt stand quite.. If the person are troubling without any reason stand in front of them and try to loud and ask what's your problem?
If someone is troubling U in a public place u can ignore them if it is possible for the 1st attempt.
If not, u can firmly warn them not to disturb u unnecessarily.even then if they are not listening .U can take the help of the other people at that place.If a bunch of people will come to help u,I am sure that person won't trouble u again.The extreme step can be to call police & ask for help.