you can define a new security rule for the "Purchase/User" group and restrict access to certain objects or actions. Here's how you can do it:

1. Go to "Settings" in the Odoo backend and enable "Technical Features" under the "General Settings" tab. This will give you access to the "Access Rights" menu.

2. Navigate to "Settings > Users & Companies > Access Rights" and search for the model for which you want to set access rights (e.g., "Purchase Order" or "Purchase Order Lines").

3. Click on the model to view the access rights for different user groups.

4. Click on "Create" to define a new access rule. In the "Object" field, select the model for which you want to define the access rule.

5. In the "Group" field, select the "Purchase/User" group. This access rule will apply to users in this group.

6. In the "Rights" section, you can set the different permissions for the "Purchase/User" group. For example, you can allow "Read" access and disallow "Create," "Write," and "Delete" access.

7. Save the access rule.

Now, users in the "Purchase/User" group will have restricted access to the specified model based on the access rule you defined. They will not inherit access from the "Purchase/Administrator" group.