It is difficult to say how reasonable it is that someone could have convinced Macbeth not to kill Duncan. On the one hand, he seems extremely reticent to kill him before his wife convinces him to. On the other hand, as the play goes on, the audience sees the deep well of ambition and bloody thoughts within Macbeth. To convince Macbeth not to kill Duncan, I would probably appeal to his faith in the witches. They told him he would be king and put the thought in his head in the first place. So, it stands to reason that they might be the key to stopping him from making a horrible mistake.

I would point out to Macbeth that although the witches said he would become king, they did not tell him to commit murder. I would fib and make the case that the witches are obviously part of the spiritual, natural world: their other proclamations have already come true naturally. I would tell him that if he tries to interfere with his own destiny, he will probably end up putting himself off course: especially since Malcolm is next in line for the throne anyway. Convincing Lady Macbeth would be a smart move, too, since she holds so much power over her husband. But if I failed to convince Lady Macbeth, I would perhaps try to use her own emasculating tactics against her and tell Macbeth that it is unmanly to be so swayed by what his wife thinks.

One could attempt to convince Macbeth that killing King Duncan will only lead to more bloodshed and anxiety, which can be psychologically damaging and stressful. With Macbeth as the illegitimate king of Scotland, his position and title will be in danger as Malcolm and other loyal followers of Duncan attempt to dethrone him. Macbeth will then be forced to defend his position as king by murdering his opponents. This increased anxiety and stress will be psychologically harmful as Macbeth will be forced to eliminate his enemies.

One could also make a religious appeal by informing Macbeth that committing such a heinous crime will have spiritual consequences. Killing a king is not only a crime, but is also a serious sin, which Macbeth will have to atone for in order to save his soul. Essentially, Macbeth risks corrupting his soul and suffering for an eternity in Hell if he kills Duncan.

Given The Witches' prophecies, one could also argue that Macbeth will not have a legacy even if he does attain the title of king. The witches predicted that Banquo's descendants would become kings, which reveals that Macbeth will not have a lasting legacy.

Simply that seeing as he has put his faith in the witches, and he has become Thane of Cawdor 'without his stir' as he himself says, there is absolutely no reason to suppose that the kingship will not eventually fall to him. And also, once Malcolm is named Prince of Cumberland, Macbeth has no reason to hope that he himself would be named as king following the death of Duncan. Of course, he manages to convince the other thanes that Malcolm's flight means that he must have arranged his father's death (which also makes no sense).

As both previous posters mentioned, it would be difficult to talk Macbeth about of this. I think the best chance you'd have to convince him to not do it would be to convince his wife. Because she's the one who continually goads MB into this action, she's the one you'd have to try and reason with. Unfortunately, her over-reaching ambition would that difficult, too, but perhaps looking into the future and giving her examples of what bad things may happen as a result of this murder would have been enough for her to put on the brakes.

Perhaps the easiest way to convince Macbeth not to kill King Duncan would be to play on the doubts he already has. In his soliloquy, the speech delivered in Act I, scene vii, Macbeth admits that this assassination will bring with it severe consequences. In addition to damning his soul, Macbeth realizes that this violent act will lead to other violent acts, some in retribution for the king's murder. In addition, Macbeth realizes what a dishonor he would be doing to the man that has just rewarded him with the additional title of Thane of Cawdor. As his king and as a guest in his home, Duncan should be protected by his host, Macbeth.
At one point Macbeth goes so far as to call the whole thing off, but is once again spurred on by his wife. Someone close to Macbeth could probably convince him to abandon his murderous scheme for all of these reasons, plus the fact that the witches never explicitly say that Macbeth must murder Duncan in order to be king when they prophesy in the opening scenes of the play.

First, let me say that I don't think I could. By the time Macbeth kills Duncan, he's already too far gone.

That said, if I had to try, I would try the following.
1) I'd review what Duncan had said about Macbeth (all the positive things).
2) I'd review how Macbeth mistrusted the witches at the start, and what a generally bad idea it is to act on prophecy.
3) I'd review all that Duncan had done for him.
4) I'd review his duties as a soldier, and try to hold him to those.
5) I'd predict the future: that killing a king is a bad precedent, especially if you want to be king.
6) I'd try to get him to wait, hoping for more rationality to return.