I sat next to a woman on the train once who had her bag on my seat. It was only narrow - perhaps 3 inches wide, however the seats were narrow and as there was no room left in her seat it completely took up space on my seat so my seat was effectively 3 inches narrower.

I politely asked her to move it and she just ignored me. A minute or so passed, and then I gently took her bag and placed it on the floor close to her. She immediately grabbed the bag and rammed it back in place. I then took it again and placed it on the floor. At this point she exclaimed 'It's just a f**king bag' and picked it up and stormed off to another carriage.