There are a couple of options here:
Do not manually add exercise ever.
If you have a step tracker that is synced with your MFP:
- FREE VERSION: Go to More --> Steps --> Do Not Track Steps.
- PREMIUM VERSION: Go to More --> Goals --> Fitness Goals --> Exercise Calories --> Off.
NOTE: With the Premium Version, you can still see how many steps you had, but it won’t adjust your calorie intake. If you do NOT have premium, the only way to stop this is to turn off step tracking completely.