Life doesn't just die without a reason. So maybe there's a rolling cataclysm that happens every couple of hundred years which wipes out most life on the planet. And the only thing stopping it might be your humans. But, as Ian Malcolm was wont to say "Life finds a way."
For conditions to be right to sustain humans, there has to be the right mixture of oxygen in the air. Before life on this planet, the air was not breathable for us. Plant life is what changed things and made it sustainable. For a rock in space to have an atmosphere, the right mixture of oxygen and all that sort of thing...you really do need life, it doesn't spontaneously happen.
If your colonists are growing things, there has to be, to put it bluntly, poop in the soil. And, if humans are living there, they will be contributing life and organisms to the planet.
If they are raising animals, those animals will also be contributing to organisms on the planet on a microscopic level.
That doesn't mean that conditions aren't harsh, and that there won't be very little life.
Terraforming HAS to happen in your scenario, but life is not going to just give up and die, unless what they brought with them isn't enough to sustain a population. Even then, life will develop. It could look to the naked eye to be devoid of life, but...so did the dust bowl at one point...
What you might do is look at the Precambrian era--there's life, but it isn't complicated, and will give settlers what they need in terms of oxygen. However, that life will not disappear once your settlers are dead or gone. It will continue to adapt and eventually become complex, but that will take...oh...billions of years.
But, happy news in this article you can make it earth. That's right. Earth.
There's a little thing called The Lomagundi Event that happened here on earth. Prior to that the oxygen levels were pretty low, but suddenly, on a massive scale, they spiked. (Could it be your humans? Using a plankton soup in the oceans to push the oxygen levels up to a level they could use?)
“The Lomagundi Event has recently been proposed as an interval of rather high oxygen levels, perhaps even nearing modern values,” says Michael Kipp at the University of Washington in Seattle.
There is no consensus on why our planet briefly gained and then lost
an oxygen-rich atmosphere.
And during this time, evolution may have been bumped a little, but it soon dipped back down again. Because so too did the oxygen level. And nobody really knows why.
It's called an oxygen oasis in time.
TLDR: Don't make your planet devoid of life, just make it uncomplex life, and the small population of animals the humans bring with will die off, leaving complex life to continue to evolve over billions of years, but certainly not present a thousand years after they leave, when the planet's artificially rizen oxygen levels plummet. Look into the The Lomagundi Event.