http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151...
However, my understanding is that these have such slow rates of growth that they might not be terribly good at "enriching" the Martian atmosphere, at least in their current natural form.
Not many things could destroy both Earth and Mars, except the expanding of the Sun in 5 billion years and maybe the fusion Andromeda/MilkyWay in 4 billion years if we're not lucky. Also aliens.
Edit: interesting to read: http://en.wikipedia.org/wiki/Terraforming_of_Mars
(Dolphins and some others seem smart, too, but they wouldn't venture to other planets.)
You could try to introduce a weakness to a particular poison (one that is not actually poisonous to anything you might want to introduce later), but that would be selected out against pretty fast. And genedrift would probably remove it even before you actually introduce the poison.
A vital dependency on some specific substance wouldn't work either. For one thing, you'd have to provide that substance Mars-wide, and also there would be a strong selection pressure to get around that dependency.
You could try to come up with some other timing methods. But genetical engineering is not magic.
You could build the architecture of the genome such that any minor mutation to the "death gene" immediately kills the organism. You could probably get it to the point that it would require $n$ simultaneous point mutations to disable the "death gene" without killing the organism. What you'd really want to worry about is whole chunks of genome being deleted (either through a serious accident in reproduction or a virus). You'd want to design the genome such that each part checks the others parts (hell just use checksums SHA3 is available now), if any part comes up bad, the organism dies.
tl;dr It should be possible to freeze evolution (or just freeze particular gene's) using modern cryptographic techniques.
The latter might not be a problem on Mars. But in such a harsh environment you do not only have to worry about competition, but about surviving at all.
And while you might kill 90% of a population, or 99% or perhaps even 99.9%, you won't kill them all.
Hmmm. Kind of a bummer Monday morning thought.
If something happens and they do evolve fast enough to pose a threat, perhaps they will evolve fast enough to realize that two planets with intelligent life are better than one.