There is no way I've found to "lock" the D200 settings to solve this. However, I've learned personally that "technique" will fix this problem. I found when I was shooting too fast, this was when the settings drifted and I didn't notice. Remember, we have all the info in the viewfinder so if we're taking our time, we are more likely to see the settings aren't what we originally desired. Digital has caused a lot of us to just shoot away, because it's free, and see what comes out the other end. Some call this the "spray and pray" method.
Please don't misunderstand that I am assuming this is what you're doing and I know this is a different answer than I'm sure you were looking for. But when I've read, or spoken with more accomplished photographers than myself, every one of them advocates slowing down and taking more time to get better shots. I've found this includes looking at the info in the viewfinder and that all my settings issues have gone away. Not having a settings lock has forced me to slow down and for me this is a good thing.