Do you mean that you want to remove an end device device from the Zigbee coordinator? The device should be removed automatically from the router table automatically after a timeout period. Did you want the device to notify the network that it is leaving and should be removed before it leaves?
My apologies for the long delay before my reply in this case.
In current version of the Zigbee stack there is no API to perform removal of device that cannot be communicated. As a workaround, please call zdo_mgmt_leave_req when device to be removed arrives the network (in ZB_ZDO_SIGNAL_DEVICE_ANNCE signal). This is not an optimal solution since DEVICE_ANNCE is a broadcast message and could be missed by the coordinator, but in lack of other API this is currently the best solution.