This is a utility function, meant to be used inside a Greasemonkey script that has the "@run-at document-start" directive set. It Checks for and deletes or replaces specific <script> tags.

checkForBadJavascripts.js

/*--- checkForBadJavascripts()
    This is a utility function, meant to be used inside a Greasemonkey script that
    has the "@run-at document-start" directive set.

    It Checks for and deletes or replaces specific <script> tags.
*/
function checkForBadJavascripts (controlArray) {
    /*--- Note that this is a self-initializing function.  The controlArray
        parameter is only active for the FIRST call.  After that, it is an
        event listener.

        The control array row is  defines like so:
        [bSearchSrcAttr, identifyingRegex, callbackFunction, shouldRemove]
        Where:
            bSearchSrcAttr      True to search the SRC attribute of a script tag
                                false to search the TEXT content of a script tag.
            identifyingRegex    A valid regular expression that should be unique
                                to that particular script tag.
            callbackFunction    An optional function to execute when the script is
                                found.  Use null if not needed.
            shouldRemove        Should script be removed when found                    

        Usage example:
            checkForBadJavascripts ( [
                [false, /old, evil init()/, function () {addJS_Node (init);} ],
                [true,  /evilExternalJS/i,  null ]
            ] );
    */
    if ( ! controlArray.length) return null;

    checkForBadJavascripts      = function (zEvent) {
        for (var J = controlArray.length - 1;  J >= 0;  --J) {
            var bSearchSrcAttr      = controlArray[J][0];
            var identifyingRegex    = controlArray[J][1];
            var shouldRemove    = controlArray[J][3];

            if (bSearchSrcAttr) {
                if (identifyingRegex.test (zEvent.target.src) ) {
                    stopBadJavascript (J,shouldRemove);
                    return false;
                }
            }
            else {
                if (identifyingRegex.test (zEvent.target.textContent) ) {
                    stopBadJavascript (J,shouldRemove);
                    return false;
                }
            }
        }

        function stopBadJavascript (controlIndex,shouldRemove) {
            zEvent.stopPropagation ();
            zEvent.preventDefault ();

            var callbackFunction    = controlArray[J][2];
            if (typeof callbackFunction == "function")
                callbackFunction (zEvent.target);

            //--- Remove the node just to clear clutter from Firebug inspection.
            zEvent.target.parentNode.removeChild (zEvent.target);

            //--- Script is intercepted, remove it from the list.
            if (shouldRemove) controlArray.splice (J, 1);
            if ( ! controlArray.length) {
                //--- All done, remove the listener.
                window.removeEventListener (
                    'beforescriptexecute', checkForBadJavascripts, true
                );
            }
        }
    }

    /*--- Use the "beforescriptexecute" event to monitor scipts as they are loaded.
        See https://developer.mozilla.org/en/DOM/element.onbeforescriptexecute
        Note seems to work on acripts that are dynamically created, despite what 
        the spec says.
    */
    window.addEventListener ('beforescriptexecute', checkForBadJavascripts, true);

    return checkForBadJavascripts;
}


function addJS_Node (text, s_URL, funcToRun) {
    var D                                   = document;
    var scriptNode                          = D.createElement ('script');
    scriptNode.type                         = "text/javascript";
    if (text)       scriptNode.textContent  = text;
    if (s_URL)      scriptNode.src          = s_URL;
    if (funcToRun)  scriptNode.textContent  = '(' + funcToRun.toString() + ')()';

    var targ = D.getElementsByTagName ('head')[0] || D.body || D.documentElement;
    //--- Don't error check here. if DOM not available, should throw error.
    targ.appendChild (scriptNode);
}