I agree tanking teams need severe drops in attendance(like 5000 attendence in a 20000 capacity arena) or a double demotion(From NBBA to D.III).
I'd even say 5k is high for a team like that. Would you even pay to watch Kobe and the Lakers take on a team of high school girls who was 0-20? If so, would you pay $170 for a courtside seat?
Maybe a solution is to view the league average salaries. For instance, average salary in the NBBA at this moment is still around $452k/week. Now, this includes teams like Bulls & LMA who spend $800k+ every week, but it also includes the following:
Team weekly salary
Wake Forest 12,574
Ice Storm BC 17,028
Southsound (bot) 20,787
King Drive Ballers 99,751
Salisbury Auerbachs 239,748
I know Edju isn't tanking (as he's recently sold a couple players), but I threw his number in there for a reason.
Imagine using a team carrying >50% of the league average salary as a threshold for tanking. This would put the current threshold in the NBBA at $225k, which keeps Edju's current salary safe from penalty as long as the other three keep their salaries low. Each time any team bought or sold a player, the threshold would raise or lower accordingly.
The only problem I can immediately see with this idea is newly created teams' salaries in competitive lower divisions (i.e. IV and below) where the average salary might be nearly double what a newly created team may have (or can afford due to small arenas and STH).
A method of curing this is to use CrazyEye's suggestion of merging this information into the Fan Survey. For instance, the line "The team manager is working hard to improve the team" would strictly penalize high level teams with hordes of money and no salaries (KDB has something like $18M, I think), and not penalize the newly created teams. If you have nothing in the bank, there's no penalty, but if you have $6M+ (just as an example) and are losing every game, your fans wouldn't be buying tickets, but burning merchandise and hanging you in effigy in the parking lot; attendance and merch income should suffer greatly in these cases.