Since SAP CRM 6.0, a billing transaction type is alwasy displayed in the follow up transacton type slection list for an sales document.
It is a functionality change, that is intended for the convenience of creating billing document, the billing document appears as an default entry in the follow up transaction type selection list.
If there is the business requirement to get rid of this option, you need to enhance the view and deactive the relevant codings in the enhanced class CL_BT115H_S_SOHOVERVIEW_IMPL method EH_ONFOLLOWUP
*** Add billing on popup
lr_context_node = gr_proctype_bo_popup->get_context_node(
iv_cnode_name = ‘PROCTYPE’ ).
IF lr_context_node IS BOUND AND me->is_template( ) = abap_false.
CREATE DATA ls_struct_ref.