How do I remove / destroy Planet Defenses?

Open up the player.entity file for the race youw ant (in this case the TEC) and delete the line that says Entity "Gauss Defense" (not forgetting to reduce the line count by one of course).

And voila, youre player will never be able to build gausse platforms any longer!

I assume you mean your own defenses.

it's the same as for all the ships. there is a 'scuttle' function, the button is located under the unit window. so basically in the middle of the screen (horizontally speaking) and at the very bottom. it sort of looks like just an aesthetic UI part, but it's really a button.

you even get a small refund if you scuttle stuff.

Shadowhal is right. When you have something selected there's a secret little button underneath the unit portrait to scuttle it. That will destroy the thing and give you a refund.

For planetary defenses, the planet also has an upgrade that increases the amount of defenses you can have. So if you haven't researched that, you could keep your guass platforms and add hangars (and repair bays!) to them.

Shift+S is the hotkey for that (or Alt+H if you're using an old version, it was a ninja change!)

yeah, that Alt+H always made my computer beep because windows didnt like what i was pressing.... idn