I have connected the telephone line from my Vigor 2860VM+ to my PSTN line and have setup two SIP accounts with phones attached to each of the VoIP ports on the router.

My issue is how do I stop incoming PSTN calls from ringing the SIP VoIP phones?

I could disconnect the telephone line, but then I wouldn't be able to route calls from the VoIP phones to the PSTN line if required.

I wish to use the VoIP lines for buisness purposes whilst retaining my private PSTN for personal incoming calls.

Is there a simple solution to this?

Is suspect that it's not possible. The "failed" (no power) condition is to connect the phone directly to the POTS line - the idea being that if there's a power cut, you can still make calls. That is electromechanical, not software controlled so that it can work without power... so when it's idle, the phone is connected to the line. To stop that, the relay would have to be permanent energised/held open which isn't good design, I'd guess.
Most people would want to receive those incoming calls.