It would seem your main method to stop tanking is restricting the number of players allowed in each zone so therefore restricting the formations usable. A different approach to the problem in terms of formations might be to restrict the amount of players by dividing the field into 3 sections lengthwise and not allowing more than 5 players in each of the left and right sections and no more than 6 or 7 in the middle section. This would still allow you to play any normal " non tanking " formation. I would also make it so you cant play a player out of position if you have an available player in your squad that could fill that position. A way around this I guess would be to only buy players of 1 position so I would add that if you cant fill your formation at the start of a match then you forfeit and receive nothing for the day , no daily token , no friends bonuses and no free stuff either. Tanking would pretty much cease at that point I would guess but ..............
In regards to the ability to fill your squad with much lower star rated players maybe a minimum allowable star rating could be applied. This rating would be dependent upon which level your playing at and would immediately diminish the ability to tank somewhat in my opinion. Quite simply you cant use what you cant buy.
Basically if you have to set your players out in a usable formation and you cant fill your squad with substandard players then your ability to tank would be severely limited , still possible to tank however but at least a few ways to do it are closed off.