I dont know what they could do besides have a few game tanking penalty towards your attendance. It clearly has to be something financial. Although I'd prefer it if the non-tanking teams received a benefit instead of the tanking teams receiving a penalty.
The solution is actually really, really easy.
(1) Ramp up the effects of 30+ pt losses on future attendance.
(2) Ramp down the effects of close losses.
(3) Extend out the duration of these effects so that they last more like 4-6 games.
This both provides a motivation to keep it close and a disincentive to completely tank. And it also fixes the problem of the immediately preceding road game having undue influence on home game attendance (by smoothing out the effects over a larger window).