Go to Settings → Account → Users & roles. Click New role to create a new role, or click an existing role to edit it. In the Set sales role access table, set the permissions as None under the Change and Delete columns. If you also see a Set marketing role access table, do the same there.

1. Under the Change or Delete column, hover over the role setting you want to change
2. Click the dropdown that appears
3. Select the access type:

For Company, Contact and Opportunity, the lowest setting under Change is Only own. Users with the role can still edit their own companies, contacts and opportunities. Document has no Change setting, and Campaign and Report have no Change or Delete setting.