You can edit the access rights for "mail.message" model

In Odoo 15, to restrict the ability for users to delete and edit chat logs, but still allow administrators to do so, you can use the Access Control List (ACL) feature. Here are the general steps you can follow:

1. Go to the Settings app and open the Access Control List page.
2. Create a new access rule, and select "Mail.Channel" as the object.
3. In the access rule, set the permissions for the user group that you want to restrict. For example, if you want to restrict all non-administrator users, you can set the permissions for the "Employees" group to "Read Only".
4. For the administrator group, you can set the permission to "Full Access".
5. Save the access rule.

This way all the users will be restricted to only read the chat logs, and only administrator will have the power to edit and delete the chat logs. Please note that these steps might vary if you have any custom modules or customizations to the default odoo15, and you may need to adjust the permissions accordingly. It's also important to test it in a test environment before implementing it in production.