CENTRAL_BACK error messages in Table SXMSPHIST

Hi,

In table SXMSPHIST there are 3643 history messages of type CENTRAL_BACK. I want to delete these messages. All these messages are in system error status (I cross checked from message monitoring in Runtime workbench). However these messages are not seen in SXMB_MONI. So there is no way I can cancel these messages manually. I ran the report RSXMB_CANCEL_MESSAGES but still no use. Can anyone please tell me How can I remove these CENTRAL_BACK messages from table SXMSPHIST? This is urgent as the DB table is growing quickly.

Thanks.