We went to the theatre once where, just before the show started a man came in, respectably dressed, and sat in the seat in front of us. However it soon became very evident that, despite looking neatly dressed, he cannot have had a bath or changed his underwear in living memory.
By the interval the smell was ruining the show for us and the people sat on one side of him got up and walked out partway through the first act. At the interval we spoke to the Front-of-house staff and explained the situation and asked if there were any empty seats elsewhere. Initially they said 'no', but then checked and found two uncollected tickets in the box office and moved us to those seats. Had these seats not been available we too would have had to walked out.
The man concerned could obviously afford to pay for a reasonable seat in the house and had an interest in opera and was ostensibly well dressed. But whether he had something like Aspergers Syndrome or other problem that inhibits social skills I do not know but it is very difficult to know what to do when these occasions arise.