There is no constitutional prohibition on it either. In fact, since you have read the Constitution, you're aware that it's quite vague (nearly silent, in fact) on the question of prosecuting anybody.
Also, this final paragraph in Section 3 should be of interest to you, and to Mr. Trump:
Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.
That should put an end to Mr. Trump's assertion of "COMPLETE & TOTAL PRESIDENTIAL IMMUNITY".
As I said earlier, there's no need to amend the Constitution.
That is a prohibition. And he was not convicted, by the way.