This news current and posted on 'Rumours and News' (why on rumours and news ? ) , big punchup with two guys, all over a woman walking around barefoot in the cabin is probably relevent to this discussion. If there was a definate rule about not boarding in barefoot/flipflops and walking around barefoot, this may not have happened. The cabin crew should be able to enforce these rules, and staff at the gate should also be able to forbid people to board in inapproapriate attire.
Rowdy passengers in brawl on Ryanair flight