If you know where they are based you can write anonymously directly to the Inspector of Taxes at their local tax District, and include as much information as you can about what / how they're doing it.
Make sure you clearly identify the company in question; their address, who is calling the shots & making the payments etc...
Just beware that if some of the employees are drawing the dole while working, they could well be landed in it too (I have no problem with this, but if you have a friend working there you might end up feeling lousy!)