I implemented a secure logging system once, where we really wanted to be sure we have the logs and that they cannot be erased via an online attack.

What we did was use a serial port, but with no protocol (just plain ascii) and with the return wire physically cut. There was hardware flow control (RTS and CTS were there), but you could only send data one way. The logging machine at the other end wasn't connected to any other network.

You want security, you have to deal with physical security.