How to "unlink" work items in a cross project
One answer
Do you want to "break" the tracking behavior? You should be able to edit the Links on the item and remove the Tracks/Tracked By link.
If you want the item off of the plan, you will need to plan it for a different iteration (or if your administrator has enabled the feature, delete the work item). As long as the work item matches the Team and Iteration configured for the plan, it will continue to appear there.
You could try just resolving the item, too -- that would make it not appear on the plan any more if resolved items are filtered out (but if you unfiltered resolved items, it would be visible again).