A recent study was conducted, to research whether people would use negative SEO to harm competitor’s rankings on Google, in an effort to better their own ranking. Astonishingly, 60% would do it, 10% Undecided and 30% that wouldn’t. But it makes a point; if they could, they would harm your rankings. Negative SEO typically involves obtaining links to your competitors with bad reputations. Luckily, Google discounts these links as they aren’t reciprocal!
Personally, I wouldn’t worry. Google is pretty much aware of this practice, and there are no signs of it becoming an effective strategy in the near future.