The "kind" parameter belongs in the tracks block, not in the captions block. Nevertheless, I have the same problem. Whether setting "default" to false or omitting the parameter, the captions are on when the player starts up. You can fix it by adding
playerInstance.setCurrentCaptions(0);
after the setup. In some of my configurations this prevented the player from obeying a pause command (by Javascript), so I had to do this:
var s = 0;
playerInstance.onPlay(function() {
if(s==0){
playerInstance.setCurrentCaptions(0);
s=1;
}
});
The s variable makes sure that the captions are not turned off on subsequent pause/play actions in case the user turns them on. I assume you are using jwplayer version 7. My captions are .srt, by the way.