The bottom line is, we don't know your league. You do. What do you think will work?
In my league, I had an owner Week 1 essentially forget to set his lineup...played Peyton Manning and Foster (both who were clearly out).
For the entire week, he was the subject of a very humiliating poll question on the league site. Problem taken care of...people have been surprisingly active since.
That won't work in every league, but something will. Did you talk to the owner yet? Have you tried sending a memo that inactive owners won't be invited back next year? Did you try putting to a vote a midseason rule change?
There are many options aside from just starting to fine people or changing the rules midstream on your own. You just have to know what is going to get through to your league. If nothing does - make sure you change the rules next offseason and maybe replace an owner or two. In our league (partial keeper), we have a 'Toilet Bowl' bracket that all the non-postseason teams go to. You actually have to win a toilet bowl game or two to move up in the draft order, thus tanking at any point doesn't really do you any good.