I was looking for a few stories in which the MC is a boy who's trying to stop a girl from loving and going to a boy who's seriously bad news. Maybe he's a notorious two-timer and cheats relentlessly; maybe he's the type who views women as disposable and always discards the broken-hearted girls without any concern after a brief bit of fun; maybe he's abusive or even sadistic and will deliberately try to hurt her; he might even be capable of rape and/or blackmail. Then there's also the more "action" types, where the girl's crush is a legitimate "villain" who's got his own unique goals and purpose and will either use the girl to further his goal or attack her if she gets in the way. The point is though that the hero in question, whether he knows the specifics or not, knows that the guy is seriously bad news and is desperately trying to stop the girl from pursuing a relationship with him. He doesn't even necessarily have to be clearly interested in her romantically, though it's a definite plus in my book.
I can think of a few examples, but these examples are short-term pieces of larger works. For instance, in Nurse Angel Ririka SOS, Ririka's crush Kanou at one point after a presumed death comes back brainwashed by the enemy. Her childhood friend spends several episodes of severe mistreatment from Ririka on account of his attempts to prove the guy's gone bad. There's also a short piece from one or two series where the hero starts out jealous that his crush is going after a guy, then overhears that guy talking about really dangerous stuff with his friends (I believe in one case the guy's friends were looking forward to him following his routine of seducing the girl, taking photos for blackmail, then sharing her with them). Whatever it is, whether the guy is in love with the girl, crushing on her, or just sees her as a friend, he knows the girl is trying to start a relationship with someone who will break her heart or worse, and due to the level of interest she has in said guy she can only see his warnings as hateful slander.