Is there a cudaminer difficulty parameter option I try to mine some btc using cudaminer on cex.io.
I'm using cudaminer for that. I have a lot 'low' rejected shares due to low difficulty (On the ghash.io they say the minimum stratum difficulty is 16).

I think my rejected shares is due to this difficulty but I dunno if I can change that on my side to avoid rejected shares.

Any idea?
No. Difficulty is handled server-side rather than client-side.

Different pools have different policies regarding difficulty. Some pools have a fixed difficulty. Others automatically calibrate the difficulty depending on your hashrate (this is called vardiff). And some let you set the difficulty, on the pool website. But in any case, it is ultimately the stratum that sends out and accepts work, so it is the pool that dictates difficulty.