how can I disable everything inside a form using javascript/jquery?

This is quite simple in plain JavaScript and will work efficiently in all browsers that support read-only form inputs (which is pretty much all browsers released in the last decade):

var form = document.getElementById("your_form_id");
var elements = form.elements;
for (var i = 0, len = elements.length; i < len; ++i) {
    elements[i].readOnly = true;
}

With HTML5 it's possible to disable all inputs contained using the <fieldset disabled /> attribute.

disabled:

If this Boolean attribute is set, the form controls that are its descendants, except descendants of its first optional element, are disabled, i.e., not editable. They won't received any browsing events, like mouse clicks or focus-related ones. Often browsers display such controls as gray.

Reference: MDC: fieldset