[Phoenix-D]

To be fair any starship can render a world uninhabitable. For referece the rock that took out the dinosaurs generated about 5 x 10^23 Joules (100 million megatons) of energy when it hit; detonating all the nuclear warheads on Earth would give you less than a million.

For example a ship just 300 feet long, moving at ten percent of the speed of light generates an order of magnitude more energy than that impact. If you get up to the speed of light and -then- hit you're talking about 450 million megatons of force.

Assuming your engines can't get you to speeds that high in normal space, things aren't much better. A Super Star Destroyer is quoted at having a length somewhere around 8000m. Ramming a planet at normal orbital speeds with something that big generates around a million megatons. Your only defense against a really desperate opponent is to have enough planetary defenses to totally destroy the ship before it hits.