Unfortunately, if the league doesn't have inning limits, transaction limits, etc, then you basically just have to deal with it.
I would not suggest "churning" to prevent him from doing it. I know several people who have had their Yahoo ID's deleted for churning. Sure, they just create a new ID, but you get the idea. Then again, if this is a private league, it's up to the commish as to what is acceptable and what isn't.
Best bet is to make sure your offense is on top of their game and win as many pitching cats as possible.
If this is a private league, then I'd suggest getting an inning limit, transaction limit, etc. Something to stop the "quantity over quality" tactic.
Yes doctor, I am sick.
Sick of those who are spineless.
Sick of those who feel self-entitled.
Sick of those who are hypocrites.
Yes doctor, an army is forming.
Yes doctor, there will be a war.
Yes doctor, there will be blood.....