The flashider is "Splined" to fit the barrel and is held in place by a castle nut that has an allen head set screw used to kep the castle nut from backing out while firing.
First, back the allen set screw out.
Second, using a set of M1A/M14 castle nut pliers loosen the castle nut until it can't be loosened any more.
Third, take a leather/plastic mallet and tap the flash hider off the threads.
You may need to keep loosening the castle nut as you tap the flash hider off the threads until the flash hider comes off the threaded portion of the barrel.
You don't say what model AAC suppressor have.
Regardless of model, I'm sure you already know that you will need to use an adapter to move from the .595 X 32 thread pitch on your barrel to the 5/8 X 24 thread pitch on your suppressor/suppressor mount to get your suppressor to work on the M1A.
Castle Nut Pliers http://www.brownells.com/.aspx/pid=8...TLE_NUT_PLIERS
Delta P 5/8 X 24 adapter http://www.deltapdesign.com/accessor...muzzle-adapter
You will also need to move your front sight (if your going to use it) from the front sight base on the flash hider to the front sight base on your adapter.
A word of caution reference the use of these thread adapter's, you need to make sure that your muzzle is square before you attach the adapter.
If your muzzle isn't square you run the very real risk of getting baffle/endcap strikes on you can.
When I mounted mine (unbeknownst to me) my muzzle wasn't square.
The POI shift on my rifle (with the can attached) was about 2 feet low at 100yards.
The muzzle wasn't square and I was getting end cap "rubs" when the bullets were leaving the can. I was lucky I didn't destroy my can.
I had a local gunsmith square teh muzle and now everything works fine.
Good luck with your project and post some your suppressed M1A when you get it all squared away.