Disable Copy/Paste into HTML form using Javascript
Source:  http://stackoverflow.com/

Code:

// Register onpaste on inputs and textareas in browsers that don't
// natively support it.
(function () {
    var onload = window.onload;

    window.onload = function () {
        if (typeof onload == "function") {
            onload.apply(this, arguments);
        }

        var fields = [];
        var inputs = document.getElementsByTagName("input");
        var textareas = document.getElementsByTagName("textarea");

        for (var i = 0; i < inputs.length; i++) {
            fields.push(inputs[i]);
        }

        for (var i = 0; i < textareas.length; i++) {
            fields.push(textareas[i]);
        }

        for (var i = 0; i < fields.length; i++) {
            var field = fields[i];

            if (typeof field.onpaste != "function" && !!field.getAttribute("onpaste")) {
                field.onpaste = eval("(function () { " + field.getAttribute("onpaste") + " })");
            }

            if (typeof field.onpaste == "function") {
                var oninput = field.oninput;

                field.oninput = function () {
                    if (typeof oninput == "function") {
                        oninput.apply(this, arguments);
                    }

                    if (typeof this.previousValue == "undefined") {
                        this.previousValue = this.value;
                    }

                    var pasted = (Math.abs(this.previousValue.length - this.value.length) > 1 && this.value != "");

                    if (pasted && !this.onpaste.apply(this, arguments)) {
                        this.value = this.previousValue;
                    }

                    this.previousValue = this.value;
                };

                if (field.addEventListener) {
                    field.addEventListener("input", field.oninput, false);
                } else if (field.attachEvent) {
                    field.attachEvent("oninput", field.oninput);
                }
            }
        }
    }
})();


To make use of this in order to disable pasting:

<input type="text" onpaste="return false;" />

* I know oninput isn't part of the W3C DOM spec, but all of the browsers I've tested this code with—Chrome 2, Safari 4, Firefox 3, Opera 10, IE6, IE7—support either oninput or onpaste. Out of all these browsers, only Opera doesn't support onpaste, but it does support oninput.

Note: This won't work on a console or other system that uses an on-screen keyboard (assuming the on-screen keyboard doesn't send keys to the browser when each key is selected). If it's possible your page/app could be used by someone with an on-screen keyboard and Opera (e.g.: Nintendo Wii, some mobile phones), don't use this script unless you've tested to make sure the on-screen keyboard sends keys to the browser after each key selection.