To disable the ability to add products in sales in Odoo 12, you can use the following steps:

1. Go to the "Sales" menu in the Odoo application, and click on the "Sales" sub-menu. This will open the sales dashboard, where you can view and manage your sales orders.

2. In the sales dashboard, click on the "Settings" menu in the top-right corner, and then select the "Configuration" option. This will open the configuration settings for the sales module.

3. In the configuration settings, go to the "Sales" tab and scroll down to the "Orders" section.

4. In the "Orders" section, you will see an option called "Allow adding products in the sales order". By default, this option is enabled, which allows users to add products to sales orders.

5. To disable the ability to add products in sales, simply uncheck the "Allow adding products in the sales order" option and save the changes.

After you have disabled the "Allow adding products in the sales order" option, users will no longer be able to add products to sales orders. They will only be able to select from the existing products that are already in the order.

If you need to re-enable the ability to add products in sales at a later time, you can simply go back to the configuration settings and check the "Allow adding products in the sales order" option again. This will allow users to add products to sales orders once again.