I have had a few difficult patients in my day.What I do:
1.Talkto them calmly
2.Try to see it from their point of view.After all,their foot is still in pain or they did not get the desired result.If you can honestly say that maybe they do have a point,a quick apology along the lines of:I am sorry I could not help you the last time Isaw you.Let me see what I can do for you now.
3.If you are trying to reason with them and they are very nasty,You can refer them to the podiatrist down the block.