Hello @John2175
If you are hosting remotely you need to have CCTV at your property so you can monitor who is coming onto your property. This will help you identify whether more guests are staying at your property than have booked.
You also need to tighten up your house rules.
Say something like 'only guests who have booked and paid may come onto the property including onto outdoor areas including the pool'.
You also seem to have misunderstood how Airbnb reviews work. Reviews are left blind, so they can't leave you a bad review in retaliation of you leaving them a negative one.
Leave an honest review mentioning that when you arrived to attend to the pool you found 15-20 guests using your facilities even though only 6 had booked and paid to stay. Leave them a thumbs down.
When these guests leave check the property throughly and if there is any damage you can make a claim through the airbnb host guarantee. You need to lodge the claim before your next guests arrive.