Easy way: Increase the salary floor.
Well ... yes and no. Done just that simply, it would kill new teams.
But done only for demoting teams, well, that might be a possibility worth exploring. Maybe it can be done so that legitimate demotions are not penalized but tanking is ... if Marin can settle on a definition of tanking, which isn't so easy as he points out. For example, maybe a demoting team has a new salary floor that is somewhere between that of the division he just left and the division he now enters.
I like making the fan survey into a powerful force (and an accurate one) better. I think there are some real possibilities in that area, not just to address tanking but also in a positive way.