How to disable box "ALL COURSES" in right bar register?

Actually this is the widget that you have added on the register page sidebar. Please edit the register page and check which sidebar is shown on the page. Then go to appearance->widgets->select the sidebar of register page and remove the widget you do not want to show.

Ok I checked your site and even change the code manually in your site to add the main sidebar . It seems the issue is related to caching .Please clear your caches and even deactivate the caching plugin adn then check the sidebar on that page .

I cretaed a new sidebar named "register_page" in your site and set this sidebar in your register page settings . you can put your widgets in this register_page sidebar or can leave it blank if you want to disable it .