It wouldn't surprise me if part of why your coworkers complain about them is because they are disrespecting themselves when they decide to play friends and accept gifts.
The rich clients get away with it and will keep acting the way people allow them to act. If no one ever calls them on being rude, they may not even realize that they are being rude. (Or whatever type of asshole behavior they are exibiting)
It's one thing as a business to have a kind and friendly approach to customers, but there's a line of self respect that can easily be crossed.
It's okay to kindly confront customers behavior, you're actually showing them that you respect them if you do.
If I was the manager there, I'd probably have a conversation, something like,
"Hey, welcome in. Can I talk to you about something? We really appreciate your business, and hope we can keep bringing you nice wines for a long time to come. But, can you help me with something? Can you tune down the attitude when you're interacting with our staff? You're probably not meaning to do so, but it comes across quite rude at times."
Or something like that, depending on any exact behaviour I could point out.