Nicking people stuff isn't what Hacker News is about. I've had articles I've sent hours writing and issued DMCA takedown notices, just as the same as if someone stole my code.
To take down a site using your stuff, you use a DMCA takedown notice.
- That doesn't mean you're abusing the DMCA
- That doesn't mean that the party that nicked your stuff is in the right.
UPDATE: to those discussing individual works, the issue is that the icons are not individually similar but rather that the whole set copies design elements from the other.
If FlatUI has picked more sources to be inspired from, they'd be in a better position.