We have a case open to consider this feature for a future release. Please up-vote this question (not this answer) to show your support for adding this feature.
This is not changeable in the UI in FogBugz7. For now, the only option is to remove the relevant rows from the BugRelation table directly in the database. This requires direct access (i.e. locally installed FogBugz). Whenever you make changes directly in the database, make sure to back it up first.
Note: adding and removing subcases to a case is possible. This answer concerns the see also links created when you type "case XYZ" into a case.