I had the same problem with my ECHO 1 M249 Para.
http://www.airsoftcanada.com/showthread.php?t=120587
They are pinned with a little screw in the flash hider as Tys mentioned.
Remove the screw first.
The flash hider is probably also held in place with some sort of epoxy glue.
I boiled mine in hot water and used pliers to un-screw the plastic flash hider once the glue was heated to the right temperature.