Hi,
I hope you have inserted following code in your custom report of segmentation basis.
DATA:
lr_top_mgr TYPEREFTO cl_crm_mkttg_mgr.
*** first of all get the root mastergroup by given guid
lr_top_mgr ?= cl_crm_mkttg_mgrs=>if_crm_mkttg_factory~find_by_key( gv_root_mgr_guid ).
IF lr_top_mgr ISBOUND.
*** remove perhaps existent members in the complete tree
lr_top_mgr->if_crm_mkttg_object~reset( ).
ENDIF.
Regards,
Naresh