If Stuffit Expander insists on opening the zips you make with the Finder’s new Archive command, select said Zip file and choose Get Info. Then turn down the “Open With” panel and click the popup menu and choose “Other” Now, click your hard disk in the left sidebar, then select System, then Library, then CoreServices and finally “BOMArchiveHelper” Press OK. Then in the Get Info dialog, press “Change All” and ok that.
