Hi,

1- Restricting Groupable Fields in Rows/Columns

       <record id="view_model_my_pivot" model="ir.ui.view">

  <field name="name">my.model.pivot.restricted</field>
  <field name="model">my.model</field>
  <field name="arch" type="xml">
    <pivot string="Custom Analysis" default_group_by="[['category']]" >
      <field name="category" type="row"/>
      <field name="user_id" type="col"/>
      <field name="amount" type="measure"/>
      <field name="count" type="measure"/>
    </pivot>
  </field>
</record>

      - <field name="category" type="row"/> :-  This field will be used for rows in the pivot table. It implies that the pivot table will group data based on the "category" field of the model.

      - <field name="user_id" type="col"/> :- This field will be used for columns in the pivot table. It implies that the pivot table will organize data horizontally based on the "user_id" field of the model.

2. Removing Unwanted Options from ‘Group By’ & Hiding “None”

        - To hide the “None” group in a pivot view in Odoo 18 Community Edition, you need to ensure that the fields used for grouping (rows or columns) never have null/False values in the dataset being visualized. Odoo shows "None" automatically when grouped fields are empty.

*  Add a domain to exclude nulls:-

      Edit you action as,

             <record id="action_my_model_pivot" model="ir.actions.act_window">

                     <field name="name">My Pivot</field>

                     <field name="res_model">my.model</field>

                     <field name="view_mode">pivot</field>

                     <field name="domain">[('my_group_field', '!=', False)]</field>

            </record>

Hope it helps