You need to set the return value of setInterval to a variable within the scope of the click handler, then use clearInterval() like this:
var interval = null;
$(document).on('ready',function(){
interval = setInterval(updateDiv,3000);
});
function updateDiv(){
$.ajax({
url: 'getContent.php',
success: function(data){
$('.square').html(data);
},
error: function(){
clearInterval(interval); // stop the interval
$.playSound('oneday.wav');
$('.square').html('<span style="color:red">Connection problems</span>');
}
});
}