I suppose you could play with:
http://www.zen-cart.com/downloads.php?do=file&id=882
... and see if there is a way to limit the customer's purchase if their purchase history (order history) is less than "1".
You have the "catch-22" situation of not knowing the order history until they login, so if a new shopper starts to fill a basket BEFORE registering, and they fill it to over $500, then they are not going to be very pleased when they try to register and checkout to be told that they are trying to spend TOO MUCH and they will have to REMOVE items from their cart.
You will probably lose 99% of new customers who fall foul of this. And they will never return to your shop.
Unless there are really sound reasons for restricting a person's cart total, it is NOT a good idea.
That sounds hypocritical, as my module is designed to do just that - but I needed it because a client of mine had their card processing company place a limit on the permitted transactional value per order (which was £5000). The site was VERY CLEAR about this to all customers.