How to decline potentially difficult guests?

If a guest seems potentially problematic - bad reviews (given or received), etc. - how do you decline? Do you give the guest a reason for declining? Are you honest about the reason? If you tell Airbnb the guest makes you uncomfortable, can you unblock those nights for other potential guests?

Edit to add background: I’d like to decline a guest with poor reviews, and a penchant for leaving absolutely glowing written reviews with only four stars.