1) I think you're underestimating the difficulty of accelerating any massive object to relativistic speeds, 2) doing so with enough accuracy to hit a relatively small moving target many light-years away is not exactly trivial, 3) a civilization with the capability to accelerate a massive object to relativistic speeds is probably also capable of building self-sufficient artificial habitats around its star, which is a lot of targets you'd have to take out for complete obliteration.

So in summation, I think defending against an incoming relativistic rock is at least as easy as using relativistic rocks to obliterate a distant civilization.