I wasn’t, but I have the Googles so I’ve looked it up. 
I do read a lot of science fiction and NASA docs.
There’s two things wrong with this; one, you’re projecting human psychology on unknown species, which is not necessary or desirable, and two, it’s not necessary to “tug colonies out of orbit” in order to knock them into Earth’s gravity well.
I’m looking for examples to explain that second point; hmmm. Military vulnerabilities are often comprehensible through classical martial arts; the goal is to spend the least possible resources to achieve the greatest possible effect. Using the enemies’ energies to your own advantage instead of spending your own is key, and massive orbital habitats sitting at the top of a steep gravity well are vast potential energy waiting to be released; they’ll be traveling well over 17,000 mph. Russia, China and the USA can put low-mass kinetic weapons traveling in the opposite direction into orbit easily (my father worked on K.E.W.-2.5 during the Reagan administration, this isn’t hypothetical).
Ability to produce large effects from small inputs is titanically increased if you’re already far from the system primary and large planets; you won’t have to fight a deep gravity well as your first step. So the easiest way for a starfaring culture to wipe out a dangerously nasty planetbound competitor is to start a chain reaction with a very small push very far away. Depending on how far advanced their math is, It’s possible that by pushing a small comet very slightly in the Oort cloud, the Vogons could slam a larger comet into Charon, destabilizing the orbit of Pluto and disrupting the Pluto/Neptune resonance. We can’t solve the math for what happens next - at least, we can’t solve it yet - but some possible outcomes are pretty extreme, including things like Venus** and Earth literally colliding. Putting more targets in Earth orbit isn’t militarily sound; a single planet is already absurdly vulnerable, and that’s just making it worse. We really don’t know how good it is possible to be at planetary scale billiards.
If, instead, we keep orbital structures relatively small and low mass, and build our major space habitats at L4 and L5 where they won’t require constant active orbit management, we begin to lay the groundwork for the InterPlanetary Transport Network.
** Or Mercury, but almost certainly not Luna.
