No, it's never justified.
If you truly believed the artist is some horrible person that deserves it, then what would you be doing in the audience? 99 times out of 100, your participation is money in their pocket. Even if there's multiple artists in the bill. If you have a moral objection, don't show up, it's that simple.
This isn't hypothetical for me, given that I dropped an artist I loved for as long as some of you have been alive. Do you think I'm going to any event where their on the bill? No, I'm withdrawing any form of financial support whatsoever, and extending that exclusion to anyone who even associates with them. That is the natural reaction a fan would have to an artist turning out to be a terrible human being.
But it's not about that when it comes to black oceans is it? It's just an excuse to bully someone. While it never manifested in reality, there were some loud people suggesting that vcha should get a black ocean because... *checks notes* .. they opened for Twice? And I believe the Xikers guys actually got boos when they opened for Ateez pre-debut (though I maybe wrong about that). This isn't people doing it because they believe the artists are "problematic". They're doing it because they want to be mean, to be spiteful bullies.
And frankly, even if it is just because the artist is "problematic", it's still not justified. There's this modern idea that being a bully to someone is righteous if that someone "deserves" it. There's so many examples of this, it happens almost daily on social media. But what it actually is, is people wanting to vent their aggression on an "acceptable" target. In a perverse way, this is seen as doing something good. It isn't. Being mean isn't "praxis". Actually contributing positively, in whatever sphere you're interested in, is what real activism is.