A no-TV day during sweeps would definitely have an effect. And broadcasters would hate it. Their ad rates are based on viewership at specific points in time (months of February, May, and November). If you were to even just severely cut back your viewing during those times, it would hurt them badly. No, you wouldn't get everyone to agree to do it, but a large number could have a disastrous effect. There is no making up for not watching during sweeps by watching at another time. Doesn't work that way in TV.