I've searched the forum but found nothing.

How do I delete a test case or test plan from RQM?

In the View Test Case/Test Plan list tables, just select it and click the red X icon.

If you don't see the read X icon, then it's a permissions issue. The role that your user account has been assigned in your project area doesn't have the "archive" permissions. The permissions of interest would be:

Quality Manager->Save Test Case->Archive
Quality Manager->Save Test Plan->Archive

Here's more info on roles and permissions:

http://publib.boulder.ibm.com/infocenter/rqmhelp/v2r0/topic/com.ibm.rational.test.qm.doc/topics/c_permissions.html