Hello Ahmad,

I think the best way and easiest way to achieve what you want is to create a tickbox on the contact model using: 

class odoo_help_5(models.Model):
    _inherit = "res.partner"
    ignore_purchase = fields.Boolean("Not a vendor")

Then add this to the contact form view, tick admin and my company and any others you don't want to be displayed in the vendor Many2one. 

Then just change the domain by using this code to inherit the purchase order form view:

<xpath expr="//field[@name='partner_id']" position="attributes">
  <attribute name="domain">['&amp;', ("ignore_purchase", "=", False), '|', ('company_id', '=', False), ('company_id', '=', company_id)]</attribute>
</xpath>

You could change the domain to ("name", "!=", "Administrator") etc however I think avoiding hard coding contacts to the domain would be a better approach.

I hope this helps.

Thanks,