Following is a post from AR15.com that is being reposted with the author's permission:
Originally Posted By RamblinWreck:
I got tired of looking at the blocks in my 2 Colt prebans, and when I couldn't install RRA 2-stage triggers because of them, they had to go. I have done this twice now, and if you are very careful, you won't scratch your lower, at least not on the outside. The drill it/Marvel mystery oil/hydraulic method has been proven to be bunk by a few that have tried it, besides, whenever you push the blind pins out it creates an ugly raised lip on the aluminum at the edge of the hole. My method leaves the pin in and preserves the stock exterior look of the lower. PS these lowers only had one blind pin thru the side; I have seen some with two and even three blind pins thru the side of the lower - this won't work on those - you'll have to mill those out. Here's how I did it:
Strip the lower internals and find some duct tape. Using a Dremel with a new cut-off disc (buy the little jar full of them - you're going to need a lot of them), I first CAREFULLY make a thin cut thru the block close to the edge of the lower, and then a second one down thru the raised portion of the block - you need clearance here for the Dremel shaft to make the 1st cut as deep as you can. I used a bit of Kroil as a cutting agent and went slowly - I still broke a lot of wheels.
1st cut is started on right, 2nd cut thru the raised portion almost done.
Make a sideways cut to remove the piece of upper block.
Cut the thin slice as deep as you can, use some tape on the lower so the Dremel shaft doesn't mar the finish. You want to cut thru and well below the blind pin. I then cracked off the thin piece by bending it back and forth. Once this is done, grab the block with a big pair of channel lock pliers and wiggle it until the inside blind pin (a roll pin) breaks and the block can be wiggled forward and out. All done.
Pin is still in, looks stock.
Good luck. HTH