Hi Diego!

The approach you are suggesting is not ideal - let me explain why:
The create button in the kanban is not the only place where users can create opportunities from → they can come from a different view (e.g. leads), they can try importing the records or they can create a new opportunity through sale order in debug mode:

Normally when you need to restrict someone from Creating a record in Odoo, you create a new Access group (res.groups), where you define the CRUD for the specific record. As it will prevent the users from creating opportunities whatsoever, they won´t be able to create them from the "alternative" places either.

Hope my answer helped!

Sincerely

Ivana from Edoo.is