As long as collections reward tradable assets (MT coins or cards), then farmers are always going to find a way to profit off those rewards by completing collections multiple times across multiple accounts.
EA's Ultimate Team modes had that same problem for a while, until they realized that you can only put an end to farming by making the collection rewards impossible to transfer from one account to the other.
This is a good solution to the problem, but it's a shame that Visual Concepts waited this late in the lifespan of 2K15 to correct it. Farmers were already ruining the economy of this mode on release day, and they have continued to do more damage over time as more people realized how easy it is to trade MT between multiple accounts.