Prior to JIRA 3.2, workflow steps using the default "Closed" JIRA status had the special behaviour that disabled certain operations such as "Edit". Since 3.2, this behaviour has been made available to all workflow steps, regardless to status. You can disable issue editing on a workflow step by adding a meta attribute "jira.issue.editable=false" to the relevant steps.
For backwards compatibility, all workflow steps with the "Closed" step that is stored in the database have been upgraded to have this new meta attribute. So for the majority of users, JIRA will have already done the upgrading for you. For those with workflows that are stored in the file system, you will have upgrade manually (if you want the closed status to disallow edits). Instructions are below:
Upgrading file based workflows
First, find the workflow step with the status id of 6 (closed)
Simply add the following to the step:
Restart JIRA and all the new changes should take effect.