I should be able to mark a link value as unselectable

(User Story, Investigate , Priority: Medium, Test Status: No automated tests yet , Reported By Justin du Coeur, )
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Summary: That is, I can add a Boolean property which, if set, prevents this from being selected in drop downs, tag prompts, and so on. The idea is that this is still a valid thing, but should not be used in anything new.
Requested by Aaron. He traditionally includes an "Enabled" flag on his reference rows in MySQL, specifically to be able to do this. It seems like a reasonable across-the-board feature, and probably has to be built in to work properly.