Currently when you create a branch based review, Objective section of the review is filled from all commit messages in the branch.
Are there a ways to control that behaviour?
Yes, you can define the default objectives at Crucible Administration > Projects > Edit Crucible Settings > Default review objectives. If this is set, then the branch review won't add the commit messages in the objectives field.
If you don't want to set anything to the review objectives at all, you can just insert an empty space character as a default review objectives. I understand though this is merely a workaround, so I've created the improvement request https://jira.atlassian.com/browse/CRUC-7231 suggesting the change on this feature. Please feel free to vote for this, as well as adding yourself as a watcher to it to track its progress.