Overview
Unlinking cases, or removing referenced cases from the "See Also" section is not currently available in FogBugz.
Once a case has been linked to another case (for example, by typing "case 1234"), the reference to that case will remain in the "See Also" section, even after removing the text by editing the case event (comment), or even after completely deleting the case comment.
Information
Providing the ability to unlink cases is on our product team's backlog. If you would like to upvote for this feature so we implement it earlier, please let our Support Team know.