...people would still correctly call him "Mr President" after he's removed from office? There's two schools of thought about it:
In the United States
The 1787 Constitution of the United States did not specify the manner of address for the chief executive. When George Washington was sworn in as the first President of the United States on April 30, 1789, the administering of the oath of office ended with the proclamation: "Long live George Washington, President of the United States." No title other than the name of the office of the executive was officially used at the inauguration. The question of a presidential title was being debated in Congress at the time, however, having become official legislative business with Richard Henry Lee's motion of April 23, 1789. Lee's motion asked congress to consider "what titles it will be proper to annex to the offices of President and Vice President of the United States - if any other than those given in the Constitution." Vice President John Adams, in his role as President of the United States Senate, organized a Congressional committee. There Adams agitated for the adoption of the style of Highness (as well as the title of Protector of Their [the United States'] Liberties) for the President. Adams and Lee were among the most outspoken proponents of an exalted presidential title.
Others favored the variant of Electoral Highness or the lesser Excellency, the latter of which was vociferously opposed by Adams, who contended that it was far beneath the presidential dignity, as the executives of the states, some of which were also titled "President" (e.g. the President of Pennsylvania), at that time often enjoyed the style of Excellency; Adams said that the President "would be leveled with colonial governors or with functionaries from German princedoms" if he were to use the style of Excellency. Adams and Richard Henry Lee both feared that cabals of powerful senators would unduly influence a weak executive, and saw an exalted title as a way of strengthening the Presidency. On further consideration, Adams deemed even Highness insufficient and instead proposed that the Executive, both the President and the Vice President (i.e., himself), be styled Majesty to prevent the "great danger" of an executive with insufficient dignity. Adams' efforts were met with widespread derision and perplexion; Thomas Jefferson called them "the most superlatively ridiculous thing I ever heard of", while Benjamin Franklin considered it "absolutely mad".
Washington consented to the demands of James Madison and the United States House of Representatives that the title be altered to "Mr. President."
In past years, some guidebooks on manners maintained that in the U.S., the title should be reserved for the incumbent president only, and should not be used for former presidents, holding that it was not proper to use the title as a courtesy title when addressing a former president. Despite that, all living former U.S. Presidents continue to be addressed as "Mr. President", both formally and informally, and contemporary experts on etiquette now maintain that it is entirely appropriate.
In the United States, the title "Mr. President" is used in a number of formal instances as well: for example anyone presiding over the United States Senate is addressed as "Mr. President." Other uses of the title include presidents of state and local legislatures, however only the President of the United States uses the title outside of formal sessions.
If one is removed from office, it seems logical that one is no longer entitled to be referred to by the title of that office.
This would be breaking new ground, since we've never had this situation in the past. I think that if one is stripped of office then one is also stripped of title for that office. It would be like identity theft to continue claiming the title for something you've been stripped of. That just makes common sense. Previous Presidents are referred to as President out of honor and respect. A pResident who gets removed from office for high crimes and misdemeanors should not be entitled to either.