I found the solution to the problem of RPCSS.exe opening up ports, even though they are blocked by my firewall.

It was posted by JusMe (thanks) and suggests renaming RPCLTSCM.dll (Avast can’t be active while doing this so I used Safe Mode or you can stop Avast from starting using MSCONFIG) After renaming and a reboot, RPC is still functions, but no longer has ports open to the net. ;D