Really elegant solution. I'd love to see this part of the official Twitter product.

If you wanted to go one step further, you could insert an invisible length Unicode character (or two) into the mentioned username. The troll is still able to make their point to people who choose to follow them, but you'd have to search pretty hard to find that mention, and it wouldn't be clickable. I've not thought my idea through in any depth though, so there could be unanticipated issues with this as an approach!