First I don't think there are many player who has that amount of money.
And Secondly We can limit the interest rate by like Depositing :
1 Pokemony- 999,999 Pokemony ->1% per hour
1mil Pokemony- 9 mil 999,999 Pokemony -> 0.5% per hour
10 mil Pokemony - 49 mil 999,999 mil Pokemony -> 0.2 % per...