In Project Settings ->HDRP Default settings->Default Volume Profile Asset its not possible to select NONE. After None is selected, field just reverts to a previous state.
Currently if having default postprocessing effects is undesirable, users have to create an empty volume and assign it as a workaround or click trough post checkboxes one by one.

Reproduction:
1) Project Settings ->HDRP Default settings>Default Volume Profile
2) Select None

UPDATE:
The main concern is that None option is visible and does nothing. Will confuse the users and is likely that they will report it as a bug.

Some suggested solutions:
1) Hide the None option if its not meant to be used.
2) Keep None option and when its selected assign an empty Volume profile. That way users get a quick option to disable default POST if they desire to do so.