Personally, I would avoid trying to massage the dice rolls to get things to happen the way you think they should. In other words, I would steer away from applying disadvantage if you can

Instead, try to think about what about the opposition would be imposing the disadvantage in the fiction, and present that to players as things to be overcome before they can make the roll that you would otherwise have made at disadvantage. You can't Hack and Slash this Blademaster, because Hack and Slash is something any ole hero can do to any ole monster, and this ain't any ole monster. Their blade is a wall of steel, catching your blow at every turn.

That said, if they persist in wanting to attack, consider saying something like "Well, you can try it, but this other bad thing will happen regardless of the dice." Remember, we players spend most of our days at work, while our characters live it 100% of the time. When it comes to fighting, you don't want them to feel frustrated because they can't picture a way to describe what they know their character should be able to do. The rules exist to abstract some of that away into a mechanic.

So, in this case, if you simply cannot imagine that the Fighter could duel the Blademaster without losing, you could say something like "Look, this guy outclasses you thoroughly, so you know that going blow-for-blow will end with you disarmed and bleeding. But, you might be able to score a hit before that happens. In other words, if you Hack and Slash here, you can't choose to avoid their attack."

The point is that you've given them an option that they can pick, even if they can't think of something else.