Find the nicest, most objective way you can to say it and say it without emotion and be honest.
But, in my experience, leave at least one thing out.
I had a guest who left the apartment in a complete shambles, she nagged me endlessly and arrived early, she treated me like a servant. But I probably would have given her my usual - lovely person + comment on the weather during their stay.
However, she also left half-full beer bottles in the trash, smoked in the apartment and left every single window open. She also neglected to text us when she was leaving - which is one of our rules so we know when it’s safe to enter the suite.
In short, she was the worst guest we ever had. Hands down. In the review I said, clearly and as unemotionally as possible, that the apartment looked as though she was just leaving for a few minutes and would be right back, the windows were wide open, there was rain coming in on our new floors. I also said I found a cigarette package wrapper in the bedroom. I let the reader put the pieces together themselves.
She was furious and blasted me privately, to the point where I stopped reading her messages, then she called Airbnb, who of course had a full account from me and could see that our rules were strictly no-smoking, so they knew what was up.
She didn’t get any traction. What she did do was put the review she gave us on her own page. Cut and pasted it - along with a rant. She just made herself look bad. It has had no impact on us that I can discern.
I think the thing is, give yourself some breathing room before you write the review - we typically wait until the very last minute if we don’t like a guest. And be cool, objective and just as you do in your listing - undersell. Don’t tell it all just hit the high points. Remember there are no paragraph breaks in reviews. More than a few lines and you look a little hysterical.
I also find it helps a great deal to tell Airbnb everything that went wrong in the section for private feedback. That way, they’ve got your back. Honesty, transparency and courtesy go a long way in a bad review.
That’s my best advice - only ever had to warn people off a guest about three times and I was subtle enough not to humiliate the guest in the process. I think that’s key.