I would just say to this employee. I will call him Tom.

Tom I have a problem and I need your help. He will of course say what is your problem. Now it is you turn to explain without getting hostile that you appreciate him trying to help you close sales but you would prefer it if you allow you to deal with the customer you are waiting on by yourself because sometimes the customers get confused with too many people trying to help them and confused people don't buy.

Then the next time he trys to butt in just say nicely Tom Thanks for offering to help but I think I can handle this without you but if I need your help I will let you know.

Hope that helps.