Weird...
I re-opened the project and it stopped at the end.
I think I know why it kept playing past the project end previously:-
I sometimes do not get the results that I expect when it comes to effects tails. They sometimes become cropped short instead of fading out to "inf."
To work around that I place a short MIDI note, in its own clip, about five or so seconds after the end of the project. I then mute that clip, but I know that the project will still carry on playing to that clip, even though it is muted.
When I did this in my current project, the event draw duration was set to that 15 minute long note, which was the last event that I placed. So when I placed my next note, near the end of the project, it was also fifteen minutes long. It didn't need to be that long so I edited it down to a crotchet (hey! not bad for a dumb ambient "musician!").
I think that Sonar still thought that my project finished at the end of that second note, even though I had made it much shorter.
I've just exported audio and it was the correct length this time.
No changed were made to the project at all, by the way, I just closed it and re-opened it.
cheers
andy