SetDirectory["C:\\drorbn\\AcademicPensieve"]

DeleteDirectory[#, DeleteContents->True]& /@ Select[FileNames["thumbs",".",∞], FileType[#]==Directory&]

Select[FileNames["previews",".",∞], FileType[#]==Directory&]

DeleteDirectory[#, DeleteContents->True]& /@ Select[FileNames["previews",".",∞], FileType[#]==Directory&]