It's best to report hackers in-game:

But sometimes the game ends or the hacker leaves before you have a chance to report them.

In these circumstances you can report them directly to the Dirty Bomb support team.

You should supply a link to the profile of the offending player.
This can most easily be found by either:
- Open in-game Steam overlay, clicking "View Players" button
- Open main Steam window, click "View" menu, click "Players" menu option

- Click "Recent Games" tab, find player, click "Profile" link, right click, click "Copy Page URL"

Once you've got their profile link you can submit a request to the DB team.