UPDATE: i386 package is updated, because it was not correctly packaged.

Element.implement({
	removeAllOptions: function() {
		if(this.get('tag')!='select') return this;
		for(var i=this.options.length-1;i>=0;i--) this.remove(i);
		return this;
	},

	addOption: function(text,value) {
		if(this.get('tag')!='select') return this;
		var optn = new Element('option');
		if(text) optn.text = text;
		if(value) optn.value = value;
		this.options.add(optn);
		return this;
	},

	removeOption: function(prop,value){
		if(this.get('tag')!='select') return this;
		for(var i=this.options.length-1;i>=0;i--) {
			if (prop=='selected' && this.options[i].selected) this.remove(i);
			if (prop=='value' && this.options[i].value==value) this.remove(i);
			if (prop=='text' && this.options[i].text==value) this.remove(i);
			if (prop=='index' && i==value) this.remove(i);
		}
		return this;
	}
});

// remove all options or empty select box
$('selectbox').removaAllOptions();

// add options, one after another
$('selectbox').addOption('One',1).addOption('Two',2).addOption('Two',3);

// removing options 
$('selectbox').removeOption('text','One');
$('selectbox').removeOption('value',2);
$('selectbox').removeOption('selected');
$('selectbox').removeOption('index',0);