As implied in the other thread, I make a natural psi shield work against casual scanning and such, but against an assault (whether an attack or a probe or forced communication) it would be more of an opposed check. A natural psi shield may always be lowered if so desired.
A further refinement that I have never really thought about is if there is an ability to "knock" (i.e. simply alert a shielded individual for the desire to communicate) or if shielding is "all or nothing". Can a psion with a shield keep it up against everyone but one individual? Don't know on that one, and it never came up.
For mechanical psi shields, I make those act like impermeable force fields. They can't be penetrated irrespective of psionic strength. If they are powered, they will have an on/off capability. If they are unpowered, they have to be physically removed. Of course, what is power or unpowered is an open question. Are psi shield helmets powered or unpowered? Is psi shielding integrated into a starship's hull powered or unpowered? Do both exist, or is everything either powered or unpowered?
The ability to integration psi shielding into ships and buildings brings up even more interesting questions. Take a ship with psi shielding. It obviously stops telepathy and life detection through the outer skin. Does it also stop teleportation? Does it stop telepathy within the ship? Is the shielding effect omnipresent within the area enclosed by the shell, or is it only inside the shell itself? If the latter, does it work only on the outer shell, or is it in the interior walls and bulkheads, too? Quite the rabbit hole if you want to explore it.