Hi Gabriel - we've not yet added the functionality to actually delete a workout, so for now I'd recommend just using the 'Enter exact cals' feature on that workout, and entering a value of 1 - I know this isn't ideal, but for now I'm afraid it's the best we can do.
We'll automatically purge all workouts with a calorie value of 1 when we roll out the update that allows the deletion of workouts.
Thanks, and sorry for the inconvenience.
Maxwell