I haven't had this problem on A2 stocks on newer lowers, but I had a similar problem when trying to replace the A1 stock on my Colt SP1 with a recent production A2 stock. With everything tightened up, the stock still didn't seat quite all the way against the rear of the lower receiver (and yes, I did remove the spacer).
I solved it by not solving it. I just put the A1 stock back on and left it at that for now. At some point in the future I will try again. I may have to turn the buffer tube into the receiver a little more, or shave a tiny amount off the rear of the buffer tube.
Good luck.
P.S. The A2 stock should tighten against the lower receiver even with the spacer missing (if the butt screw is long enough to reach the buffer tube. And it certainly should be long enough since it just extends through the spacer anyway). The spacer just prevents severe overtightening.