As someone else pointed out, if you don't like this then your gripe should be with the slow departments, not with the bank.

An alternative might be to allow weekly training purchases to continue, but not allow any equipment/etc while your balance is negative. This makes it so nobody misses a week of training because of a slow department, it also prevents people from spending all $12m (hypothetical) before it is deposited by the graders or whoever.

However, I'm totally behind the bank not allowing undeposited money to be spent. It can be annoying to certain individual circumstances, but for the sake of running a sim bank this size, we need to be a little more strict and organized.