I get this a lot as a retail site. There’s no way to stop them. I think you apologize that they’re having an issue and refer to them to the proper channel.
You could close the topic so it will drop off the grid. Many of our customers will search for a place to complain and post in dead topics.
There might be a way to get a message to pop up in the preview pane if they use certain words, similar to the one that shows up if you’re repeating something that’s already been posted in a thread. Not sure on that but seems like it could be done.
For my particular case, while I’m not CS, I do have access to their tools and am notified when there are issues. Often times, these posts are the first indication we get that there’s an issue with a sale. I forward these on the the proper teams. For some quick/minor issues, I help them if I can.