El present perfect continuous ya tiene un verbo auxiliar, to have, y este verbo auxiliar se utiliza para la negación.

ejemplos       He has been writing a letter He has not (haven't) been writing a letter   She has been reading an article She has not (haven't) been reading an article   We have been watching TV We have not (haven't) been watching TV   They have been building a house They have not (haven't) been building a house   You have been planning a trip You have not (haven't) been planning a trip

En el caso de que haya un verbo modal, es este verbo modal que hay que negar. No discutimos sobre la relevancia de este tipo de construcciones. Se entiende sin problemas que construcciones de este tipo son un poco "exóticas".

ejemplos     He could not have been doing his homework   No podía estar haciendo sus tareas   She might not have been reading an article in this moment   A lo mejor no estaba leyendo un artículo en ese momento   They did not need to have been watching TV to get the news   No hacía falta que estuviesen mirando la tele para estar enterados.   We should not have been partying all night instead of studying   En vez de ir a la fiesta habría sido mejor estudiar