Poster A was wrong.

Simple as that. An RP is a collaborative work to generate a fun and/or interesting story. We assume everyone as equals so to do something as "I have a plot in mind, ya'll are not to interrupt it and instead accommodate to me," sounds downright ridiculous. Who are you to dictate that you or your characters possess more importance than others? Why on earth should other people accommodate and compromise around you so [i]you[/i] can your ideal plot out?

Some might argue and say Poster B was partially at fault for not accommodating. Well, if someone copied your homework without permission then you have the right to be angry. Would it make sense if someone told you that you shouldn't have gotten angry and instead have been mature about the matter? You could have but you are not obligated to nor are you in the wrong for not letting the matter slide.

I don't think this is really debatable.