I am not sure where you are going with this one coach.
Baseball is a game of failure. It is NOT a game of not trying hard enough. If the player is trying, and giving his heart out, but not conditioned yet, then why would you punish him for that?
Now if he is quitting and can't complete the task and gives up, this is another question. You can still keep trying but not complete a task and that to me is not a problem, giving up is.
It was my observation that the best HS programs I saw were the ones that separated the fall conditioning from practice. They either did it in the morning or used the "practice time" for conditioning. The very best ones really got into strength and conditioning early as incoming freshmen and continued it through their Sr season.
I know a lot of college programs have made fall conditioning ingrained in their culture and have a competition, and team building.
Here is just one example:
https://www.youtube.com/watch?v=DAX4DhvJoAY
Good Luck with your team!