Hi Arpit Thakar,

Software users means, they are the employees of the company.

So when a user is created the same time same named partner will generate.

if you look at the customer menu you can see there is filter.

then the employees are not listed under the menu.
if need to restrict them in Many2one field the n do one thing,
just set a domain for thet field.

        domain="[('customer','=',True)

It will solve your issue.

If you need to remove the creation of res.partner from the user, its not a good practice. 

Regards

Nikhilkrishnan