In greenhopper, we can restrict who is allowed to schedule issue, but people can still go in Jira and change the fix Version if they have permission to Resolve Issue. Developper can resolve an issue, but not necessarily reschedule it for a later release. A workaround is to change the workflow so that resolving is enable to specific groups but this is not very generic.

This was discussed in: https://answers.atlassian.com/questions/91533/disabling-ability-to-change-fix-version-in-jira