I'm using v11 so I may not have the exact same thing as you.
When I looked at Group Permissions, I saw a Copy option under Test, but that was only for the Test Folder. It wouldn't stop the copy of an individual test. And, like you, I did not find a Copy option in the Defects section.
I think the only way you can prevent the copy is through workflow customization. You would need to capture the actions that are triggered when a user tries to copy a defect, and then abort the process. Review the Admin Guide for info on Workflow Customization.
Use your Tools > Customization > Workflow > Script Editor to modify Workflow Script > Project Scripts > Common Script > ActionCanExecute as follows:
<font class="small">Code:</font><hr /><pre>
'Use ActiveModule and ActiveDialogName to get
'the current context.
On Error Resume Next
ActionCanExecute = DefaultRes
On Error GoTo 0
That will create a message box to display the name of the action being executed when you use the copy features. Once you know the name of the action, you can use a select statement in the same script to check if that action is being executed, and prevent it from proceeding.