Need some help setting up code so that if Product A,B,or C are added to the cart, the site doesn't allow other products to be added, restricts checkout, and a note indicates the problem.

I thought it might be doable with some modifications to Dr.Byte's code seen here:
http://www.zen-cart.com/showthread.p...t=subscription
by implementing steps 1 and 2, yet so far I have been unsuccessful. Might that post be missing something?

I am currently using Fast and Easy Ajax Checkout so that throws a loop in the mix. But I would think my restriction should be kicking in before the checkout process is even allowed to begin.