Thank you Samuli! I tried to use the following code within one of the views of HRECM00_PLANNING_UI with no luck:
data lr_lbl type ref to cl_wd_toolbar_button.
lr_lbl ?= view->get_element( ID = '_CFG_BUTTON_8' ).
IF lr_lbl IS NOT INITIAL.
lr_lbl->set_enabled( EXPORTING VALUE = abap_false ).
endif.