As sometimes it may be somewhat unintentional, I really struggle to find the words to make a rule that fits.
That said, if someone suspects another team of tanking, they need to let me know. If I think there is merit to the claim, I will take it to the TC and we'll talk about it. If there is general agreement, I will contact the GM and give them a sim to cleanup their team or else I will have to help them with some callups.
A few notes, from the top of my head, if they have talent in the form of MLB ready prospects but they choose not to call them up, that is fine. Even I wait with some of my guys to ensure I call them up for maximum usage considering their arbitration years. Anyone can tank the last sim, last two weeks, of the season. Some MLB teams already do this (like the last day of school, who gives a ****.)