If you have a chest or upright freezer put the rifle in and let it set for a couple of hours. Take it out, put it in a vise, heat the rear of the receiver up just a bit with a propane torch and then wack the buttstock with your hand or a rubber mallet from different angles and it should come out.
If not, with the rifle in the vise use a socket extension placed into the receiver up against the toe of the buttstock near the pistol grip nut (may need to remove the pistol grip and nut) and tap the socket extension with the mallet. It will come out.
Before trying to tap the buttstock out, remove the bolt carrier and the selector from the rifle and then you can get the right angle to insert the socket extension into the receiver on an angle. Do not use a screwdriver blade to do this or you will split the laminated wood. Do not pry with a screwdriver between the buttstock and receiver/rear tang from the outside or you will split the laminations (in the picture it looks like you already tried to do it)