You can mark the element with the data attribute while you're typing and unmark it when you're done. Your modified code:
$.fn.Typewriter = function(opts){
var $this = this,
defaults = { animDelay: 50 },
settings = $.extend(defaults, opts);
$this.data('typing', true); // we are typing!
$.each(settings.text, function(i, letter){
setTimeout(function(){
$this.html($this.html() + (letter!='\n'?letter:'<br />'));
if (i == settings.text.length - 1) {
$this.data('typing', null); // this is the last letter, we are done typing!
}
}, settings.animDelay * i);
});
}
$('#game_howto').mouseup(function(e) {
if (!$('#game_howto_slide1').data('typing')) { // only run if not typing
$('#game_howto_slide1').Typewriter({
animDelay: 50,
text: 'Typewriter effect'
});
}
});