I have a temporary solution which does not require hacking the plugin files. Add this javascript to your page. If you don't know how, use a plugin like "Simple Custom CSS and JS" and add as front-end javascript in footer.
jQuery(document).ready(function () {
(function() {
var ev = new jQuery.Event('style'),
orig = jQuery.fn.css;
jQuery.fn.css = function() {
jQuery(this).trigger(ev);
retu orig.apply(this, arguments);
}
})();
setTimeout(function () {
jQuery('img.ajax-loader').bind('style', function (e) {
var style = jQuery(this).attr('style');
//console.log("style: " + style);
if(style) {
if(style.indexOf("hidden") > 1) {
// enable button
//console.log("Disable button");
jQuery('input.wpcf7-submit').attr('disabled','disabled');
} else {
//console.log("Enable button");
jQuery('input.wpcf7-submit').prop('disabled', false);
}
}
});
}, 2000)
});