These answer are kinda about going from "undesirable" to "undesirable and creepy". I'm betting the spirit of the question is more what makes someone undesirable in the first place.
I know for me I'm pretty protective of my time, so almost everyone is undesirable to me all the time. But depth of conversation or an interesting approach to life will gain my attention which may become friendship. But mostly I like to let friendships die down after a while to messaging once every few months.
I usually let other people pursue me rather than the other way round since other people have higher socialization need than me, but this means I'm choosing my relationships through filtering which is kinda a sucky feeling. Also it means almost all of my friends have been other guys, which I'd kinda like to change since I would like to date, but it's really difficult.