<form id="QandA" action="<?php echo htmlentities($action); ?>" method="post">

    <table id="question">

    <tr>
    <td colspan="2">
    <a onclick="return plusbutton();">
    <img src="Images/plussign.jpg" width="30" height="30" alt="Look Up Previous Question" class="plusimage" id="mainPlusbutton" name="plusbuttonrow"/>
    </a>
    <span id="plussignmsg">(Click Plus Sign to look up Previous Questions)</span>
    </td>
    </tr>
    </table>


    </form>

In the code above I am able to replace an image with another image when the if statement is met. But my problem is that when the image is replaced, it does not disable the on click event. My question is that when the image is replaced, how do I disable the onclick event onclick="return plusbutton();?