Once used it has no more power? What kind of non-sense is that? That's like saying "once someone breaks your legs they can't break them again", which is also a false statement and grossly diminishing the severity of the situation. If blackmail can destroy or end an MC's life, then that is a not a weak weapon. The fact that you can't get destroyed twice, even though you can, is kind of irrelevant when the consequences of that blackmail are severe enough.
Also, the fact that it's illegal is also completely irrelevant, not just because something being illegal doesn't mean it can't be done but because a character would also have to prove that she committed the crime. Trying to go after someone blackmailing you "legally", is also a good way to have whatever they were holding against you used against you, or just have it backfire and used against you. It's also entirely possible for a woman to get away with blackmail, if it falls into a "her words against his", especially if it's a fantasy femdom setting.
If you had a female character, for example, tell an MC, "my dad is a big time mafia boss, if you don't do what I say, I'll tell him you raped me and he will have his people come here and cut your balls off"? Do you think it matters that it's "illegal"? Could the MC even prove that anything she just said was said? Does the fact that she said it take anything away from the threat? Do you think it matters that the threat is no longer threatening after the MC gets his balls cut off? Do you think there's nothing else that she could do to that MC after that initial threat is carried out, given that she was in a position to blackmail him in the first place?
Even looking at less extreme scenarios like "I have your DNA all over me, if you don't do what I say, I go to the police and tell them you assaulted me" or "I have a recording of you doing something illegal, if you don't do what I say, it's going everywhere", that's still pretty much life destroying. Even if the MC decides to react with violence, this only works against him and further incriminates him. Best case scenario is that he's taking someone down with him.