Reaching even a small proportion of the speed of light requires prohibitively high amounts of energy. Let's consider the energy required to accelerate Haley's Comet (relatively small at 2.2e14 kg) to half the speed of light (v = c/2):
For comparison, the Earth absorbs each year about 3.85e24 J of solar energy, so we're talking about six million years of total incoming solar.
Assuming two societies with similar technology, it will cost vastly less energy for the defender to give the object a small nudge than it cost the attacker to propel it in the first place. The attacker would do much better to invest into a fleet of ships that can respond to contingencies and adapt to the defender's response.
E = mc^2 / sqrt(1 - v^2 / c^2) ~= 1.15 mc^2 = 1.15 * 2.2e14 kg * (3e8 m/s)^2 = 22.77e30 J
For comparison, the Earth absorbs each year about 3.85e24 J of solar energy, so we're talking about six million years of total incoming solar.
Assuming two societies with similar technology, it will cost vastly less energy for the defender to give the object a small nudge than it cost the attacker to propel it in the first place. The attacker would do much better to invest into a fleet of ships that can respond to contingencies and adapt to the defender's response.