As a virologist, the behaviour of some passengers on that TUI flight was unacceptable and compounded by the lack of any sort of monitoring by cabin crew. Posts here talk about the options, what can the cabin crew do etc. It is very simple. People get asked to put on their mask or return to their seats. The cabin crew know or should know who is sitting in which seat, details etc...So ask them once to put the mask on. If they don't comply then that's it. They never fly again. Ever. Then the message will get through their thick skulls.
This is not a 'polite' virus. Tie for us to stop pussy-footing around in my professional opinion.