How do contractors handle clients who keep requesting changes after the job is done?

Work into your price that basically says, "You have me or my crew for a total of 3 hours over the next 30 days". Whatever you can get done in those 3 hours is all the extra work we will do for you.

Sometimes too, you just have to take your medicine and do and fix up some stuff. This is where pricing is very important. Make sure you have enough meat on the bone that you can go back there one or two times. If your work is quality, you shouldn't need to and your pricing should reflect the quality of your work. The support after the job is done is part of the quality.