Hi Beatriz,
I hesitate to mention it because I've just discovered right now that it seems to have a bug in the latest version, but the Checklist Marking Guide addon works the way you want. https://moodle.org/plugins/pluginversions.php?plugin=gradingform_checklist
Version 2.5 works correctly in Moodle 2.5. However, in version 2.6 for Moodle 2.6 the Item and Group delete buttons are missing when editing the marking guide so you have to use three or more items per group. What that means - When you add a new group, by default it creates 3 items. In 2.5 you can delete ones you don't use. In 2.6, since the delete buttons are missing, you cannot get rid of extras, and it won't let you leave them empty.
I've tried using a different theme to see if that was the cause, but the delete buttons were still missing. I even installed the 2.5 version in Moodle 2.6...still no delete buttons.
The only temporary fix I've come up with so far, is to make the Marking Guide as usual and in any extra item name I name it delete me 1, delete me 2. I then go find it in the database using phpMyAdmin and delete them. Seems to work ok, at least it gives the right grades.
Ah, well, probably more than you want to put up with. I'm off to do some semester exam grading but will try and put in a bug report later today.
AL