I recently bought a M&P Shield 9, and the ONLY thing I didn't like about it was the external safety. I didn't want to remove the mechanism completely, and I needed a way to disable it, so I trimmed off the little tab that blocks the striker. It was a surprisingly simple operation, and I'm 100% happy with the pistol now.
I got a little ahead of myself, and forgot to take pictures of the safety lever in stock form, but here's what it looks like now. The side opposite the external lever had a tab that extended about 3/8" inward. I cut it off with a dremel, polished it smooth, and touched it up with a sharpie.
Here's a short, crappy iPhone vid of me dryfiring it: