I get where you're coming from, I've definitely been in that position before. I don't think it's so much about being rude as it is about being firm and direct so that you can regain control of the situation. I usually end up going with a few different responses, the most common being "Thank you but I'm not interested." Then if he keeps bothering me, I keep repeating "I'm not interested" and walk away when possible. Sometimes I'll try to zone out/ "look through" the person, in hopes that they can pick up the non-verbal cues as well.

If the situation continues to be a problem, I'll say something along the lines of "I'm really not interested, and I'm not going to change my mind. Leave me alone." I've found that being blunt and to-the-point without going out of your way to offend is one of the more direct ways of getting rid of someone. I also try to avoid "I have a boyfriend" on principle (even though it's true), because it's disgraceful that so many dudes will only get off your case if some other dude "owns" you.

"Why not, you got a boyfriend?"

"It really doesn't matter, I said no." / "Do I need a boyfriend to turn you down?"

Things like this used to make me freeze up, and there were times that I let people cross boundaries I set because I didn't know how to address it or I didn't want to be rude. But as it's happened more and more, harassment like that just makes me angry. I shouldn't have to fear for my fucking safety because some douche feels entitled to my attention and his pathetic ego can't take no for an answer. Sometimes I am rude, and I won't apologize for not being a polite young lady to someone who refuses to respect me.

Especially with Merlin, being direct and then minimizing contact is probably your best bet. "I'm not interested in you, I never will be. You need to leave me alone. Don't text me, don't snapchat me, don't ask me for rides. If you want me to continue buying from you, you need to stop harassing me. Otherwise, I can find someone else to sell to me." Block the internet creep for sure. You don't owe him anything.