Got the same problem. Resolved by moving the offending change set to a new activity, and deleting the now-empty activity.
Q: When I try to deliver an activity from a stream, sometimes UCM complains of a dependency on another activity in the stream. But when I look more deeply, I cannot find a change-set dependency between the activities! Why?
When you deliver an activity from one stream to another, a hidden delivery baseline is created on your behalf in the source stream. UCM requires that all subsequent deliveries from this stream contain this baseline (and all its activities). The problem described above usually occurs when you deliver something from one development stream to another, and then later try to deliver another activity somewhere (without including the first delivery's activities). There are workarounds for this. Appendix A of Software Configuration Management Strategies and IBM Rational ClearCase contains a script that can undo an activity. Or, you can resort to moving the offending change-set to a new activity and deleting the now-empty activity that was causing the dependency.
From IBM Developerworks http://www.ibm.com/developerworks/rational/library/jul05/reader/bellagio.html