If you just want to control the ability to overwrite the reports, then that can be accomplished with the basic security levels – as long as he has Full Control, then he will be able to overwrite the report.
But if you want to control the ability to use Design Mode in a particular folder, it gets way more complicated. You can use the “Customization” tab in CMC -> Users and Groups to limit the menu options that are visible in WebI. It’s not as sophisticated as regular security rights, and a little convoluted. If you don’t want anyone to be able to use Design Mode in that folder, you can open up Customization for the Everyone group, select your folder, and unselect Design Mode in Features.