There is a couple ways to identify this -
IP of the PC/Laptop/Device that is being used, in that if an IP has multiple accounts being accessed then should be flagged up straight away
Stop the Loans feature altogether, as thinking about it, whats to stop a strong person in an association, loaning the maximum number of his best players, throw his own game, but his association guys all win, as there squad % has overtaken their opponents due to incoming loans
Carefully monitor all active Associations and checking stats etc... and ensuring any player inactive for a period of time, is removed
Any new account made, can't join an Association until a specific level, such as 3+