<<Am I correct that this is not currently possible?>> Very much correct, and thank you for expressing precisely what is needed. We need the option to specify for a shared collection that users may only add/remove items that they own. It has been on the forum before [1-2] and maybe it will be noticed this time.
 (2012-03-08) Re: Why can other people remove my docs from a shared collection? (and thereby, in essence, "unshare" the doc)