Disable Image Link After Clicking

i've searched this forum before posting but didn't find a "clean" solution to my problem.
I have an image with a link on it:

Code:

<a href="#null" onClick="addValue('bold');"><img src="images/myimage.gif" title="Some text" alt="Some text" /></a>
which adds the value in a textarea in the same page via this function:

Code:

function addValue( val ) {

var tb = document.forms['valuesform'].elements['valuesfield'];

if ( tb.value.length > 0 ) tb.value += " , ";

tb.value += val;
}

I need to disable the link after the image has been clicked,preventing multiple insertions of the same value. I've modified the link this way:

Code:

<a href="#null" onClick="addValue('bold'); this.onclick=null;" ><img ......
but while only one click is allowed,i get many JS errors in the JSConsole like

Code:

uncaught exception etc.... and i think this is not a good sign.=