In all seriousness, the possibility of harmful alien contact is quite real and should be taken into consideration by the scientific community. That said, I think it's much more important (and hopefully covered in the new documentary series) to concern ourselves with our behavior as we venture out into the Universe and seek new territory and resources. I mean knowledge.
Other starfaring intlligences may well decide to do unto us before we do unto them.
You could do it with 1960s computers and whatever Sci-fi Magic propulsion system your probe needs to get from system to system. Find a big asteroid. Accelerate it into the orbit of the planet at, oh, .01c should be plenty. What is the planet going to do, roll a natural twenty to dodge?
An asteroid with a diameter of 2 km and a density of 1 ton/m^3 weighs ~410^9 kg. Travelling at 0.01c, it has a kinetic energy of ~410^22 J, or ~100 times the current worldwide energy production per year.
Of course deflecting it doesn't require you to fully stop it, but it's closing in fast and you need a strong sideways push to accelerate it fast enough to end up with enough speed to miss the planet. If we ever see an reasonably sized asteroid with reasonable speed on a collision course, we're as doomed as if we hadn't seen it.
Besides that impending death is an excellent motivator.
.01 * the speed of light = 2,997,924.58 m / s
1kg traveling at 2,997,924.58 m / s = 6,706,166.29 mph. At that speed it has the same kinetic energy as ~10,000,000,000 kg traveling 67mph.
Since the projectiles are relativistic, there is no warning.
At any rate one would think that aliens would at least understand the concept of mutually assured destruction and realize that we would not destroy them if they were capable of retaliating in kind...
No warning + no survivors = no retaliation = no MAD.
Also, the historical evidence is that we've been dangerously close to nuclear war on too many occasions for comfort.