I am quite certain that a single iron atom would have no effect on the processes of a star. Setting aside that any given star at stage in its life is bound to have a very large number of all types of elements either through its normal fusion process (while most of the fusion will be of the hydrogen and helium variety for a normal star for most its life over time randomness will result in the fusion of larger and more exotic elements, especially iron) or from leftovers of the previous stars that provided the mass to form the star. Our sun, for instance, is 0.16% iron.

All the heavier elements that make up things like planets (metals, gases, and even up to uranium and plutonium) are the product from past stellar fusion processes. Large and/or hot stars can continue fusion all the way up to silicon and at that point iron will make up a surprising amount of the star's mass.

There's no element/molecule that we know of that could be introduced in small amounts to a star to disrupt its process in any appreciable way. Unless we want to talk about exotic forms of matter, like strangelets or something, the only way destroying a star through the introduction of outside material will necessitate just a very, very large amount something (just about anything would probably do). We're talking about a lot though. I have no idea how much or how to calculate how much iron it would take to disrupt the sun's fusion process but if it were even only 1% of the sun's mass you're talking about a lot of iron.

Again, I did some math (probably badly) and what I calculated was that a block of iron equal to 1% of our sun's mass would weigh 22 trillion trillion short tons and would be 18.5 billion trillion cubed miles in volume. That's a cube of iron 26.45 million miles on each side. If anybody plans on trying this do tell so that I can invest in mining lasers now.

EDIT: I think the reason that the hypothetical block of iron is so much larger than the sun itself is because I did not (and could not even try) to account for how the block would collapse under its own gravity and reach a much higher density (and thus much smaller volume) than normal cast iron. The actual ball (you couldn't keep it in cube form) would likely be much smaller but the above still illustrates, I hope, the gargantuan amount of material we're talking about.

Lets leave aside rate of annihilation reaction (and won't call it as an explosion, well, you know, even heaviest of thermonuclear bombs do not explode, but burn) and look at the result.
First, pressure in the core of sun is about 10^17 Pa, if our charon is filled with antimatter at pressure of ~100 atm (10^7 Pa), it will collapse like in 10^10 times, well, practically, it will become much less than 1m-¦.
So, at first, we will have to stretch star by 1m-¦ (how rude!) :D
Next, the annihilation happens. This reaction causes transformation of fermion matter (like electrons, protons, that cannot take the same place in space) to bosons (like photons). The sum of mass-energy in this reaction persists, so calculation of released energy is not important. What is important, that we take away matter from the star, making a 'hole' of size less than 1m-¦ (neglecting pressure of photons in first iteration, since it will be times less than pressure of fermions).

Well, returning to energy as a measure of going supernova, one should give star about ~10^44 Joules for it to cast off a layer.

Applying point of energy-mass to the star won't make any effect. Stars are bubbles of matter, even if you 'shift' say, like half of the Sun, it will return back, of course spitting some plasma and heavy radiation from the core, but still it will reorganize itself without blowing apart.

What causes stars to go supernova, is the change in the whole volume of the star, or at least in a significant amount of volume, making a whole thick enough layer.

Well, so far it turns out to be 'how to not blow up a star'.
However, there is a 'sci-fi' solution of blowing up a star. Just open a wormhole in the core of the star. This will lead to leaking of star matter through wormhole, making new star on the other side (on the other hand the matter will just be dissipating (exploding) with high velocities because of tremendous pressure inside the star). If the wormhole will be large enough to channel huge amount of solar matter, upper layers will collapse towards the emptied core, tear apart and throw matter in different directions, making supernova.