How to manage two different rates

Hi there. I am running an event - there will be one rate for delegates and another higher rate for vendors. We have listed both rates on the website - but a few vendors have booked the non vendor rate and we have had to follow up and then they request that they get the cheaper rate because blah blah blah. We dont have the resource to manage this. How can we manage this to avoid this? We dont want to hide the cheaper rate as this is a very attractive lead in rate - but all the while the cheaper rate is visible, we run the risk of those who are not entitled to it booking it.