Since all restaurants want to be displayed at the top of the list in their area, if they have lower than three stars then their penalty is to get them listed at the bottom of the list. This could be difficult if you have less competition in a category, but eventually will force the restaurant to do better like any rating system. Plus you can make a call and see if they are purposely refusing to accept the orders, or cancelling and calling the customer directly to avoid the charges. If they do that, they are not worth the hassle and just cancel their account and withhold their money as a punishment, but make sure that is in the signing of the contract too! Poor ethics is hard to change, because they they that is smarts and being nice to them is not fair to the other restaurants too. If they cheat you, how can they be trusted not to cheat the customer too?
I'm very fair but strict and that is the way it should be.