You simply can't. Once you have watched a lesson in a tutorial the progress bar will run up to a certain percentage. When you are done with a tutorial you can mark it manually with 'I'm Done' but you cannot remove the progress.
In my opinion Guitartricks should change it so you can manually edit everything in your progress report / My Guitar Tricks so you do not have lessons in there you don't want to.
Currently it gives too much of an idea that you are progressing with a lesson when you do not even have to work on it. Sometimes I only check out a lesson that comes up in my progress report that I haven't even learned.
Hope that helps.
JJ