Nixon resigned to escape an impeachment trial. Impeaching a former official is pointless, although I think there should be a similar mechanism where a former offical could be barred from holding office again. Aside from that, if a former office-holder’s conduct is criminal, let the legal justice system prosecute zim.
I hear what you are saying, but step back for a moment and consider the full weight of what you (and probably many others) are thinking about.
By passing a law or "mechanism" to bar someone from holding public office again that would be giving the government, which is always the "current" power to directly refuse the will of the people who are voting to elect someone they support.
As our culture, not just our county, but our culture is based upon the will of the people being fulfilled the more power you give the government to say "no" to a potential candidate the less power the citizenry has.
We already have term limits for the president. The president is unique in this limitation because that office is held by one (1) person, so 1/3 of the governmental power is in a single persons hands. I think this is the reason so many folks are Okay with term limits for the president. But going beyond just term limits we start to shift the balance of power away from the people.
I am not saying you are wrong, but this is something that we should take great care in considering before pursuing. That's my only thought.
Regards,
Doug