I just installed two Moen valves like you described and had the same problem getting the stems out. I called Moen customer service, who said they should "just pull out", but like you, I found they wouldn't budge. I mad a simple puller our of a short piece of steel channel and some screws and nuts, and they pulled right out. I then flushed the new plumbing, put things back together, and only had one kind of water (hot or cold, I don't remember which). Checked and found I had installed the stem 180 degrees off. Moen's pictorial directions (designed so that no matter what language you speak or read, you won't be able to figure out the instructions) do in fact show the proper orientation of the stem.