If you're the owner of a shared folder, you can unshare it at any time. When you unshare a folder, the files will be removed from the members' online accounts immediately and from their linked computers the next time they sync using the Dropbox desktop application. The files will be permanently deleted from their accounts and they won't be able to recover the files, even with Dropbox's restore feature.
Unsharing a folder also gives you the option to allow members to keep their own copy of the folder and its contents. If you choose to allow members to keep their copy of the shared folder, you might want to avoid confusion by telling them that the folder is no longer shared and any changes to its contents will no longer sync to each other's Dropbox.