That situation isn't actual harassment, at least not in the legal definition of the term as long as you are talking about an out of work context and these guys just approached you once.
Is being approached by a beggar harassment? Is it these individual guys fault that many little annoyances add up to be a big headache/borderline harassment? It's not like clairvoyance is an actual thing, so what are you actually blaming them individually for? And what could they, individually, ever do to prevent that situation?
The expectation you are having here is that all males should instead always be passive and never approach a female of interest because she might or might not have been approached before and thus feel harassed. Doing this "equal" would mean females should also never approach males, where would that leave us?
Tbh there is no easy solution to this because there are issues for both sides: Females getting dogpiled and males usually getting nowhere without taking opportunities leading to said dogpiling.
On a more practical note: There are subtle ways to prevent at least some approaches from happening and deflecting them somewhat effectively. Like wearing visible headphones when in public, instantly, but politely, declining the approach with body language by keeping a steady pace of urgency and slightly shaking the head.
With time this is bound to get better, at least if eroding of societal role expectations happens equally for all genders and females actively approaching males becomes something that isn't considered "out of the ordinary" due to its sheer rarity of happening. Imho that's something that very likely will require a multigenerational effort.