Collaborator is configured such that once any defect has been found in any file of a review, then no files in that review can be removed. Your only option is to delete the review and make a new rev...
One of my users has uploaded a Subversion changelist set that apparently was the wrong changelist to an existing review. There were 2 changelists already uploaded to this review. They wanted to back out this last changelist so it doesn't contaminate the review. Separating the view didn't give the kind of display that I thought it would so we could remove the last changelist.
What I would like to suggest is a future enhancement to the tool, where the last changelist could be removed if it was the wrong one (say a different ticket/Change Request was chosen by mistake instead of the same ticket that the original review was started with), even if it included files that had comments/defects against it, as long as no NEW comments/defects had been entered.