When it comes to rejecting someone usually I would focus on someone's Moon, Mars and Mercury mostly just because it indicates us how they manage to express their feelings.
Fire signs (Aries/Leo/Sagittarius) would be very straightforward, they do struggle a lot when it comes to expressing themselves because it sounds better in their heads. So they tend to hurt people (unintentionally) because they don't know how to say it.
Earth signs (Taurus/Virgo/Capricorn) are also very direct but they're very careful with words. If they ever say something that makes you feel bad, believe me... they meant for it to hurt otherwise they wouldn't have said it lol at the end of the day, it is what it is and if they don't like you, they don't but overall they don't ever say too much.
Water signs (Cancer/Scorpio/Pisces) are ruled by emotions. The least thing they would want to do is make someone feel bad so they could end up lying... or there's just lots of sugar coatting. But they could feel so bad afterwards that they'd probably start liking the person they rejected lol
Air signs (Gemini/Libra/Aquarius) are also too nice. They're used to people liking them and they like that lol so they'd probably never actually get to 'reject' someone, unless they really have to, and even then it wouldn't sound like a rejection. They do tend to friendzone people tho.
So starting off with the one I think would be super nice about it is BangChan, I mean he's a libra, he's obviously used to it lol it'd be the nicest rejection ever, somehow he would convince u that you deserve someone better and would give you reasons why and would try to stay friends. Felix and Han would also be nice about it at first and then end up ghosting u lol I.N could be the type to ghost too but not trying to be mean, just to give u space too. i can see Hyunjin getting super shy all of the sudden when you're around, kinda uncomfortable. Changbin would be straightforward and tell u that he doesnt really believe that u have a crush on him and would try to convince you otherwise by keeping you around. Seungmin would be like cool about it, he kinda feels bad that he doesnt feel the same way. And lee know with that moon in sag and mars in virgo i expect nothing less than for him to be direct, he wouldnt hang out with u as much as he used to.