Yes.They are removable and threaded just like a regular AR.
Just make sure you clamp the barrel itself and not the upper receiver. This is very important as the polymer upper cannot take the torque required to remove the flash hider.
You can buy a barrel vice from Tacticool22 or Squirrel Daddy.
While true, it's not the whole story.
The 15-22's barrel does not attach to the upper receiver the same way an AR-15 barrel does. Instead, it is inserted through the rear of the upper receiver through grooves. If you clamp the upper receiver, you can torque the barrel to the point that these grooves are damaged, which will render the upper receiver useless.
Also, even on aluminum upper receivers, it is often recommended to clamp the barrel, not the upper receiver, when torquing a muzzle break.
What part of the story did I miss?
*Flash hider is removable
*Clamp the barrel and not the upper receiver because it can't take the torque (i.e. it will be damaged)
The OP asked about removing the M&P15-22 flash hider. No aluminum upper receiver on a 15-22.
I have another question about this. What can I use to protect the finish on the barrel while using these in the vise?