Hi,

Refer to the following modules.

* https://apps.odoo.com/apps/modules/18.0/hide_chatter

* https://odoo-community.org/shop/mail-restrict-send-button-6616?search=hide+chatter#attr=943708

For more information

* https://www.odoo.com/id_ID/forum/help-1/how-to-show-chatter-for-a-specific-group-of-user-in-odoo15-207286

* https://www.odoo.com/it_IT/forum/assistenza-1/hide-employee-chatter-by-user-182609

Hope it helps

From code side, Filters out portal users or external users then Link the user to their corresponding record (because Odoo stores partner info for both users and contacts).

class ResPartner(models.Model): 
​ _inherit = 'res.partner' 

  ​def _message_partner_allowed(self): 
  ​""" Restrict @mention to only internal users (employees) """ 
  ​ ​internal_users = self.env['res.users'].search([('share', '=', False)]) 
  ​ ​return internal_users.mapped('partner_id')