How do I get opponents from spying on me?

use your spies to neutralise emeny spies. It is in the spies ection.

Or build Counter Espionage Centers on each planet. Personally i nevr have a problem with enemy spies so i don't evr build those. I find that i cn out produce my enemies ith spies so thy never have enough to send my way. Or they just don't think I'm worth looking at.   

In DL you had no method to stop spies, but then again, you didn't notice it's affects until the end of the game and saw 'techs stolen' which could have been via invasion anyway...

In DA/TA you could try to out-produce them in spies. If you're the biggest race in the galaxy, odds are you got a better economy than they do and will outstrip them there.

The other option, as CaptainYar said, is to place a Counter Espionage Center on each planet. This however is only an option for races with that tech. The Korath's tech tree doesn't contain this tech and thus are highly-susceptible to spies. Isn't stopping them from buying it from others, like the Krynn (via Super Spy) who start off with the tech.

It's a semi-decent morale building (25%) so if you just replace your morale building with one of these, you'll kill 2 birds with one stone.

I own 80% of the galaxy. I have a military rating of 340, a economy rating of 180, atech rating of 324.

No one is even close to me, everyone else is weak in all areas.

I have 3 options:

1. Elimiante the spies.
2. Eliminate all 3 remaining races.
3. Find who sent the spies and eliminate them.

I really want to know how I can find out who sent the spies. I only have 3 races I need to eliminate before I have a conquest victory. If someone sent the spies, I'll just eliminate them. It'll be a quick victory.

I have hundreds of planets I own, so It would be a bit bothersome to spend 50 hours out spying all the spies on my planets.

The thing is, at the moment I personally don't care if someone is spying on me. My military might is 3 times the power of all the 3 major remaining races combined.

I want them to stop or, I just want a good excuse to declare war on someone.

Remember, I'm a saintly race, so I must have a good excuse to eliminate

Honestly, I like being a saintly race, I don't really want to go and eliminate everyone without a good reason. I want to find who sent the spies and eliminate them.

The thing is, even if you eliminate the spying race, I don't think their spies disappear. At least, I have left spies on my planets before as I conquered the galaxy and they have never disappeared. On top of that, there is no game mechanism that allows you to identify the spying race.

Since you own 80% of the galaxy you will get a saintly influence victory. Like Burress said, you can not find out who sent the spies. And at the stage of the game you are at, trying to prevent spies will not be worth it.

You didn'y say what game you were playing. If its DA or TA you may want to at least eliminat the enemy spy so you get back the function of what ever tile the spy is on. Besides, doing so may make the AI try to get a new py mking it waste money it probably cant afford to lose.

You mean placing a CEC on each of them?

You can see what planets have spies in the window where you set taxes and such, there is a tab for espionage.

Its easier to go to the Domestic Screen and then Espionage and eliminate them that way. Just double click on the enemy spy for whatever building and POOF, he's gone. Provided of course you have some spies too.

Enemy spies do disappear after you eliminate the race that sent them. The exception is if they surrender to you. Then their spies hang around. This can be confirmed through cheat mode

Thanks for the confirmation on that. What about if they surrender to someone else? That happens to me as often as them surrending to me, and far more often than I can eliminate them completely before surrendering.