Created host with interfaces for both Agent and JMX.
Link template including JMX items.
<Monitor.>
Disable JMX on monitored system.
<JMX connector state fails, items fail>
Unlink + clear JMX template + save.
...
The JMX interface (interfaceid:10019) can now no longer be removed from the host, the 'Remove' link is greyed out and no longer has a mouseover function.
The JMX 'Availability' state is still stuck in 'Red', with the error state.

The server does not any longer appear to be attempting to query or update the JMX state since no items are linked, however it is stuck in this condition, failed and unable to be removed.