Keeping users from messing up entry

With help from a fellow user on here, we created this cool textbox which you can test here: http://www.freewebs.com/moreauctions/addtext.html

As you can see it adds a linebreak after each text is added. It is meant to create a list to be output from a form. But a user can click inside the textarea and mess up the list and enter anyway they want.

How can I make sure that there is always a linebreak between each line of text?

<input size="20" id="inputtext">
<input name="add" value="Add" onclick="document.getElementById('textarea').value+='•'+inputtext.value+'\n\n';" type="button">
<textarea rows="15" id="textarea" name="box" cols="22"></textarea>

Code (markup):

 

You can use a readonly attribute on the <textarea>. How important is it that users do not edit the contents of the textarea?

 

I was thinking of that but I realized users need to be able to delete from the textarea.

 

Wait I think this can be done with createTextRange() but I have no idea.