People who work in retail/customer service, what's something you wish you could say to customers?

I remember asking this guy once years ago, and by this point civility had left the building and I had morphed into Basil Fawlty, “what more could you possibly want, you have had an apology that wasn’t warranted, a discount which wasn’t earned and made a member of my staff hide under a table in a fetal position sucking her thumb and drifting off to her happy place. Blood? Is that what you want, ok I’ll open a vein up now for you, I’ll go to the kitchen and get a nice sharp knife just to appease you. Do you now see how ridiculous and redundant your complaints are now, we have tried everything for you and as it must be so awful for you staying here, I suggest you pack your shit and leave immediately?” Backstory, yes this actually happened. I worked as a duty manager in a small hotel, the other duty manager had left a handover about this particular guest. The receptionist in question had only just come back to work after the death of her father from cancer and he was relatively young. This guest arrived for the weekend and was very rude from the get go about his room, the food etc and just seemed to be constantly at reception for something and my colleague had dealt with him with apologies etc. I have worked in hotels for over 40 years and have come across this kind of guest who realize they haven’t got as much money as they expected and make it their mission to try and get as much off their stay. When I walked in on the Saturday morning to see my receptionist hiding under a desk in the back office telling me that this guy had been berating her from at least 7.30 am, the rage just erupted. He appeared at the desk, she confirmed it was him and before he could let go with the attitude of ‘this is a new one I can bully’ I immediately cut across him with my sternest voice and said ‘if you wish to scream at me, nothing will happen but if you wish to take the language and volume down a notch, go into the lounge and I will DEAL with you shortly” I immediately rang the owner to clear it with him of my intention. I let him wait 5 minutes and then by that time, he was literally deflated but still began his complaint which I listened to and then quite calmly I replied with the above. When he panicked as he realised that he had nowhere to go and was ruining his girlfriends weekend away, he backed down but I was a woman with a mission at this point and would I simply would not.