Most fantasy leaguers, including myself, are firmly against churning. However, how aggressively it should be policed becomes a slightly more debated issue. And their are leagues out there that feel it is perfectly fine strategic move - it's their league I guess, so they can make the rules whatever they want.
Anyway, the point is to make sure you find a happy medium for your league, and each league is different. A "fun" league, for example, charging for drop/adds may just cause inactivity. You could just eliminate waivers so that players aren't locked, but that may give an unfair advantage to the guy who works from home over the guy who lives on a plane. It may be as simple as just educating owners that's not ok, and if you see it happening you will simply unlock the churned players. Some owners don't believe (or just don't think it through) that churning is not ok.
So maybe talk to the owners and see how they'd prefer to deal with it. In the end, though, it is your decision and make sure you get a policy you feel you can enforce.