That safety is a bad design anyhow. If you let the hammer fall on it accidently, it will peen the end of the pin that you are talking about and is probably the reason yours does not want to come out. I know that is not the answer you asked for, but that has been my experience.
More to your question, the square block is wedge shaped on the inside and pushes the pin out. I would imagine that if you get the hammer backed up on a solid surface or some type of fixture, you could whack the square block with something of a non-marring nature and get it free. My problem was that the pin would stick in the out position and the block would fall completely out of the hammer. The couple of times this happened to me about 20 years ago I put the block back in, tapped the pin back in and have never used the safety since.
Curtis