If you want to stop users from creating BO’s in other companies than their default you can create a custom event on BUSINESS_OPPORTUNITY_TAB. Tick the checkbox for “New Value” in the field COMPANY to make it a parameter.
The event action could look like this:
BEGIN
IF '&NEW:COMPANY' != User_Finance_API.Get_Default_Company_Func THEN
Error_SYS.Record_General('Event', 'EVENT_WRONGCOMPANY: You may only create business opportunity with company :P1', User_Finance_API.Get_Default_Company_Func);
END IF;
END;
Observe! This event affects all users. You can limit that by other conditions if necessary.