I once stayed at the Capo Bay hotel, Protaras, Cyprus. On checking in to the hotel you are given a number that is also on the sunbeds. This meant that those sunbeds were yours for the duration of the holiday.
Although it isn't an adults only hotel, it isn't geared up for children and we only saw a few the 2 weeks we were there.
There may be other hotels that have this system, if anyone has any experience of this I'm sure they'll be on to let you know. Good luck with your search.

adults only hotel in a quiet resort with good food and entertainment

Might be a little further than you want to travel at 5 hours flight time, but, how about Luxor?

I stayed at the Maritim Jolie Ville back in 2007, and it's the only hotel I've ever stayed at where they posted a sign telling folk not to reserve sunbeds - and actually MEANT it.
Anything left on an unoccupied sunbed for more than 45 minutes - and the pool guy would collect it -towels, books, lotion, glasses and whatever, and take it to his little secure cabin. You could collect it when you came back and find a new sunbed.
Brilliant - a complete cure for the thoughtless, selfish sunbed hoggers one encounters all over the planet these days.
It worked wonderfully. I wish every hotel would do this.

The hotel is actually great, all rooms are little private bungalows in lovely grounds on the banks of the Nile. The sunsets are amazing, the food good - and interesting - and there was good evening entertainment in "Fella's Tent". There are two large pools, one a super infinity pool adjoining the Nile, the other one a really quiet relaxing area.
They don't ban children here - but there seemed to be very, very few around - indeed the whole resort of Luxor didn't seem to be a place overrun with children.
And there's lots to go see, and it has to be one of the hottest places on earth!

the hotels in lido de jeselo, you must buy a towel to reserve your place, the pool attendant would remove your own towels if placed down before 9am

Just do what we do.
If no one turns up for their reserved sun bed at about 9.30am-10am,we just remove the towels and fold them neatly and put them on a chair nearby.
Around midday someone will come looking for the bed that they reserved at dawn.
I always ask at reception when I arrive at a hotel what their policy is regarding sun beds.I then ask if it is OK if we remove the towels if no one turns up around 9.30am.They almost all say yes,so if anyone confronts us we just tell them to go to reception and complain.

Alexander the Great in Paphos. You are given your own sunbeds for the duration of the stay and it's adults only.

Just back from Riu Seabank in Meiilieha Bay Malta & they have a sign that towels will be removed fom reserved sun loungers. I was really pleased to see a hotel taking action on this selfish practice. But not adults only I'm afraid.
It's the 1st time I've come across a hotel doing this so maybe this is the way things are going..hope so!