First : Tanking in real football exists and gaining stars is similar to players who are gaining experience... it's a choice and a right for every manager and no body has the right to object it ... but it can be regulated by making it difficult or unfavorable by stop providing tokens during the tanking season(s) ...
The real problem of tanking is related to the associations (as stated by Nikolgiorgos )
It can be easily fixed by two ways ::
1- STOP SHOWING 3 AND 4 STARS PLAYERS ON THE AUCTION
2- Allow managers to join / leave an association and captains to kick members ONLY at the end of each season and before the beginning of the new league ( 2 days /48 hours only )