Can a President be impeached after leaving office due to resignation or the end of his term? Of course since he’d no longer be in office the Senate couldn’t remove him, but could they forbid him from ever holding federal office again?
Hmmm, good question. Since Congress has declined to pursue Clinton along these lines, we may never know…
No, the objective of impeachment is to remove someone from office. After he’s out, he cannot be impeached. However, he (or she) is still subject to criminal prosecution for any crimes committed while in office.
No. The purpose of impeachment is to decide whether the officeholder shall be removed from office. From the constitution:
Would an impeached President sill be referred to as Mr. President?
Or is that title no longer applicable?
An impeached President is not necessarily convicted of the crimes raised against him. e.g. President Clinton was impeached.
The government could easily forbid a former president from holding office by convicting him of a crime and sentencing him to prison time, though this is not the function of Congress.
It depends on who’s addresing him. A former President’s style is a matter of custom, not law.
It would not be formal protocol to refer to a convicted former president as Mr. President.
Of course, it would not be formal protocol to refer to a unconvicted former president as Mr. President.
Despite the universal usage of Mr. President today for anyone who has held the office, it is my understanding that under the strictest of protocols, Mr. President is reserved for the current title-holder. Former President is correct for those who have left office; President-Elect is correct between election and inauguration.
There’s probably five people in Washington who still adhere to this rule, but I’ve never heard of it being rescinded.
Doesn’t say what the purpose of it is, but it states its reach and limits:
The bolded passage indicates that removal from office isn’t the only potential effect of impeachment, and gives a reason for pursuing impeachment after an official’s having left office.
The Constitution is silent on whether Congress could impeach once an official is no longer in office. IANAL, but IMHO that means they can do it if they want.
Given that they didn’t finish up Tricky Dick’s impeachment once he resigned (at a point when it was a slam dunk), I doubt there’d be much interest in going any further in anyone else’s case.
There is some dispute about whether impeachment trials can continue after the resignation of the office holder. We debated this recently; the search engine is spinning wheels or I’d pull it up for everyone to see. 
That is the purpose. It’s the only thing that can be done after impeachment. There are no lesser actions. If the Senate votes to “convict”, the president is removed from office. They can’t vote to slap him on the wrist and let him stay in office. Once again, from the constitution:
Shall be, not may be.
William W. Belknap, Grant’s Secretary of War, was impeached for corruption, but resigned just as his Senate “trial” was beginning. The Senate went ahead anyway with the purpose of deciding if he should be barred from future office as well, but enough Senators thought the “trial” had become moot that the vote went the other way.
The Senate can vote to remove an officeholder but *without * future disqualification, as in the case of Judge Alcee Hastings, now a Congressman.
But John, given that one of the consequences that can be voted is permanent bar from future office, it can be (and has been) argued that the trial on an impeachment isn’t stymied by a resignation, but can continue on the additional issue of permanent disqualification from office.
Indeed, the House has impeached and the Senate has tried an officer who had already resigned before the vote on impeachment in the House. Secretary William W. Belknap was impeached by the House on the same day that he resigned (March 2, 1876). The House determined to proceed against him after his resignation, and asserted to the Senate in response to a plea by Secretary Belknap that the Senate trial could not proceed because of the resignation, that the actions of the House were lawful in voting impeachment and that the Senate was empowered to try the matter regardless of the impeachment.
Therefore, by precedent, the House has the power of impeaching someone who has resigned, and the Senate has the power to try the matter.
It should be noted, however, that the Senate voted to acquit Secretary Belknap, and at least some of those who voted acquittal did so because of a feeling that the proceedings were improper after the resignation.
Remember in all this that the Supreme Court is unlikely ever to rule on issues of impeachment, leaving the matter to the political give and take.
Or, on edit, what Elvis said much more succintly. 
I believe impeachment and conviction also revokes any pension the office-holder would have been entitled to.
This isn’t explicit in the Constitution (unless you think it’s implied in “disqualification to hold and enjoy any … profit under the United States”), but I think it was specifically written into the various Federal Pension Acts.
This may have come about after Nixon & Watergate. As I recall, there was some speculation that if Nixon had resigned after Congress had actually voted a Bill of Impeachment against him, he would lost his pension, office allowance, Secret Service guards, etc.