In your diet calendar, click on the activity that you want to delete - then, where it says "Increase by," click on it and select "Reduce by" instead - then reduce it by the number of hours/minutes you had recorded.
For example, if you had 1 hour of badminton recorded, then click on that and choose to reduce by 1 hour.
Someone who is busier than you is working out right now.
There will come a day when you can no longer do this. Today is not that day.