i know the feeling i get lots of chaps sending me pictures of their bits and pieces orasking me out online and i am like, i am not interested in you but they won't listen and one bloke keeps on ringing me non stop.
Tell them you're gay. If that doesn't work(and it won't) block them. If you are talking about in real life, politely tell them you are gay, and if that doesn't work, ignore and walk away or tell them "Leave me alone you fucking creep"(depending on situation). If they don't leave you alone in real life, that's harassment, and I'm sure there is someone in your school or workplace that deals with harassment. In short, be straightforward, and only be rude if they are being a jackass. Guys have no way of knowing that you are gay until you tell them, so be polite when you do. A respectable man will let you be(or may suggest platonic friendship, but they will stop flirting). Men who are not respectable don't need your respect, so it's probably okay to be a little rude to get your point across. Good luck.
If you’re comfortable with him knowing, just say that you’re gay. If not, just let him know that you’re not interested, and him perusing you would only piss you off.
You can say some of the following "I'm sorry you seem very nice, but I am not looking for anyone at this time" "I'm sorry you seem very nice, but I am not interested in men at all, please don't take this personally, I'm gay"
Whenever I am approached by men, my first resort is to just respectfully tell them I am not looking for anything right now. If they continue despite my comments that I am not interested, I will tell them that I am gay if I feel I am not in a situation that seems unsafe to do so.
Whatever you do, don't be ambiguous about it. Men can be dickheads and try multiple times to pursue someone even if you don't show any interest in them. I've tried to shake men off by giving them the cold shoulder and moving to another part of the room or focusing intently on a newspaper to look as if I'm busy and don't want to be disturbed. But rather than interpreting this as "I'm not interested in your company, go bother someone else" they take it as "I'm terribly shy and need you to coax me into being seduced by you". I'm not a lesbian, but I've received enough unwanted attention to know that repulsive men are often blind to their own repulsiveness and think they are entitled to attention. Don't look for a polite way to say No. No means No.
I appreciate that my experiences with men will be very different from a womans experiences so I am probably not best placed to give advice. But maybe tell them that you are a strict Catholic and your father has already arranged for you to meet your future husband tomorrow afternoon. This makes the male person who is bothering you think about a few things: 1. Your father is heavily involved, weirdly protective of you and already against whatever relationship you two may have. 2. You are already basically married. 3. You have some weird religion and you are very unlikely to have sex or do anything before marriage. 4. They may have to deal with your angry future husband tomorrow evening if they try anything. Unfortunately, misogyny is alive and well and the best protection that a woman has against these sort of persistent men is other men. They respect other men in a way which they dont with women (“the bro code”). Its a sad state of affairs but I reckon what I said, or something like it, is your best bet.