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)
});