Commishes occasionally have to deal with owners signing up for their leagues and then backing out. Here's my humble suggestion for the right way to back out:
Send a sitemail to the commish explaining why you're leaving -- even if the reason is "I just don't feel like it anymore" -- and apologize.
Wrong ways to back out:
1. Post a message in the league's classifieds thread announcing to all and sundry that you're ditching the league because it doesn't seem that interesting to you after all, or you just noticed some rule you consider stupid, or the league is taking too damn long to fill, etc.
2. Don't say anything. Let everyone believe you're still in the league, and then when there are 23 teams entered and waiting for you, force the commish to go back to the forums to rustle up one more owner.
As a commish, #1 always annoys me even more than #2. I usually redline posts like that just because they kill the momentum of a league I'm trying to fill. But #2 is pretty bad too.