Use a variable and call clearInterval to stop it.
var interval;
$(document).on('ready',function()
interval = setInterval(updateDiv,3000);
});
function updateDiv(){
$.ajax({
url: 'getContent.php',
success: function(data){
$('.square').html(data);
},
error: function(){
$.playSound('oneday.wav');
$('.square').html('<span style="color:red">Connection problems</span>');
// I want to stop it here
clearInterval(interval);
}
});
}