Can the Senate bar Donald J Trump from again holding the office of President?
An interesting question, and one that I think has been answered before. In July of 1797, Senator William Blount was impeached by the House. Before his trial in the Senate could begin, the Senate expelled him and he left the Capitol to return to his home state of Tennessee.
In January of 1799, the Senate finally began Blount's impeachment trial, but Blount did not appear (he was already serving as the Speaker of the Tennessee Legislature). The trial commenced any way, and Blount's attorneys argued that since Blount had been expelled from the senate in July 1797, he was no longer an officer of the government and thus no longer subject to Senate jurisdiction.
In January of 1799, after three days of exhaustive arguments, the Senate deliberated behind closed doors, then voted on two resolutions. The first one failed to pass:
That William Blount was a civil officer of the United States within the meaning of the Constitution of the United States, and therefore liable to be impeached by the House of Representatives; That as the articles of impeachment charge him with high crimes and misdemeanors, supposed to have been committed while he was a Senator of the United States, his plea [to dismiss the charges] ought to be overruled.
The second one passed on January 11, 1799, by a vote of 14–11:
The court is of opinion that the matter alleged in the plea of the defendant is sufficient in law to show that this court ought not to hold jurisdiction of the said impeachment, and that the said impeachment is dismissed.
So you can see that the Senate of the United States in 1799 already answered that question. Now, you could argue that one Congress cannot pass a resolution that is binding on a following Congress, and I would agree with you. However, we are talking about the plain meaning of the Constitution here.
The question that the Senate was trying to answer was whether or not they had the power of impeachment. Impeachment was a term from British common law which was the removal of an officer of the government from that office. If we were to decide that any citizen is impeachable, then this would turn the entire system of jury trials on its head by permitting the Congress to hold trials and punish them without the judicial branch. This would be a violation of the separation of powers.