How to prevent SQL Server service account from registering / deregistering SPN?
This doesn't make sense, as you are running the service under a domain account with no "write ServicePrincipalName" permissions. If you were running under a localSystem account or a domain account with "write ServicePrincipalName" permissions, it would be able to create the spn. Is it possible that someone/something manually created the spn?

Here's an excerpt from Degremont Michel (SQL Server Core Engineer at Microsoft) explaining how this works:

If you run SQL Server under the LocalSystem account, the SPN is automatically registered as SQL registering with the machine account that has the right to create an SPN default. So Kerberos interacts successfully with the server running SQL Server.

However, if you run SQL Server under a domain account or a local account, the attempt to create the SPN may fail. When creating the service principal name fails, this means that no SPN is set for the service that is running SQL Server. 

http://blogs.technet.com/b/mdegre/archive/2009/11/20/the-sql-network-interface-library-was-unable-to-register-spn.aspx

Here's another excerpt from msdn:

When the Database Engine service starts, it attempts to register the Service Principal Name (SPN). If the account starting SQL Server doesn’t have permission to register a SPN in Active Directory Domain Services, this call will fail and a warning message will be logged in the Application event log as well as the SQL Server error log. To register the SPN, the Database Engine must be running under a built-in account, such as Local System (not recommended) or NETWORK SERVICE, or an account that has permission to register an SPN, such as a domain administrator account. If SQL Server is not running under one of these accounts, the SPN is not registered at startup and the domain administrator must register the SPN manually.

http://msdn.microsoft.com/en-us/library/ms191153%28v=sql.105%29.aspx

Sounds like the account "somehow" has permissions that you don't know about. Maybe group policy or something similar...