You can disable the sa account. On SQL 2005, you can also rename it if you like (and create a new login named sa with any permissions you like), however you cannot modify the original sa login.

If you try to remove it from the sysadmin group, you get an error:

Error 15405

Drop member failed for ServerRole 'sysadmin'

Cannot use the special principal 'sa'

You can rename from object explorer in management studio. Right click the login and choose 'rename'