I've been playing some MOO3 lately and one thing has been really annoying me, namely the AI keeps trying to build things on its own. I have unchecked planet econ AI and still it queues op DEA's on its own. Is there any way to completely shut it off, so that nothing gets built automatically?

IIRC, turning off the AI will prevent it from rearranging the spending sliders, but it doesn't completely prevent it from queueing other things (especially if you have any kind of development plans active).

In general you can deal with this by designating DEAs when you colonize the planet, although if you have "Natural" or "Balanced" as your overall development policy then the AI will often do a better job of choosing what's needed. (Someone analyzed the behavior of those AI settings on the MOO3 forums a long time ago, and when it comes to things like mining/bioharvest/industry, it actually does make logical choices even though the math might look odd to the player at first glance. The downside is the propensity to stick government DEAs everywhere if unrest starts to build.)

Yeah, also with regards to planetary improvements just make sure you obsolete those little and outdated ships that you don't want him constantly building.

I'm starting to learn how to work around the AI instead of fighting with it, thanks for the advice. He he. It sure can be frustrating but I haven't quite reached that point yet.

Moo3 with proper (fanmade) patches addressing bugs (not even mods) is an amazing game, its just not Moo2,5 that everybody wanted, kind of like what HoMaM4 is to HoMaM3 but more extreme.

More OT, as Garran mentions, regional zoning setting is the only direct way to tell AI what DEA to build.
After you get some tech, "(empire) development plans" are very nice tool to get what you need on most of the planets. (AI prioritizes improvements over building DEAs to fulfill its needs, so the deeper in the tech tree you are the less AI will interfere (unless you have some big shortages/ unrest problems). After some time it will take longer for it to fill region slots, allowing for more control.

It's best to micromanage only the most important worlds for example to get maximum production capacity on one planet to build big stuff fast. With proper techs, dev plans and a little pre-planning DEAs (only several, not whole planet at once) AI rarely did get in the way of micromanaging specialized worlds.