I have a samsung led60 tv and samsung hw-j250 soundbar. i connected them wiih the optical cable that came with the soundbar. the soundbar operates, but the tv speakers are still on even though I selected audio out from the setup menu.
some case are like what ssddx said. also some cases you just have to turn the volume all the way down to 0 on the tv. my older tv that's all you can do is turn it to zero as it doesn't offer option to disable its speakers.