var str = prompt('Введите строку: ', '');

function checkSpam(str) {

	var newstr = str.toLowerCase();
	var spamPos = 0;
	var spamPosLength = 0;

	while (true) {
    	if ( ~newstr.indexOf('stupid' || 'goof' || 'fat') {
    		set spamPos = ( ~newstr.indexOf('stupid' || 'goof' || 'fat');
    		set spamPosLength = (  );

    		set str = ( str.slice(0, (spamPos - 1)) +  )
    	} else {
    		break;
    	}
	}

    return str;
}

alert(checkSpam(str));