Create requirements before being allowed to take out max loans. Because seriously, a bank won't just give out $300,000 to whoever walks in. A player has to earn the bank's 'trust' to take out higher loans.
A player can't take out a loan until they pay off their initial $75 debt. When they do they can take out a loan for $500-$1000. If they pay that off then the player will now have the option to get a loan for $1000-$5000. If they pay off a loan in that range then they can take out a loan for $5000-$10,000, and so-on. Or whatever amounts and increments you want to use.
This will slow down the join-getloan-give-reset thing and reduce it's effectiveness.
Also I think it should also be a given that players with bad criminal records can't take out high loans.