I think you can fire the Governor and that will stop growth purge all population buildings might cause them to die down to one. The Yor are simple just use them for constructors they are not going to increase. I think I remember reading that you can land a colony ship on the Yor planet then put the last Yor on one last constructor. 

I however have a better solution for future games when you start one deselect the Festron, Yor, Mimot, and the Race that is a consolidation of all races.

If you do this these races will likely not show up in the game.

There is one thing that Frogboy needs to insure in the update though that if these races are excluded from being selected they do not show up in an event.

There is/was a way to ban these races from the selection process however I forget exactly how Maybe if Old Spider or Publius remember it they can jump in.  Be aware if the do each time the game updates you will have to redo it Unless someone is kind enough to explain how to make it a Mod.

2 options - Colony or transport ships.  I do that when I have a whole world full of Yor and got 0 growth.  Colony ships to populate whatever little planets and transports to continue my all-out assault on the galaxy.

Then, when enough are gone, my colony ships full of Mimot, human, anything but Festron fly in and boom! Re population

It seems reasonable to put the unwanted population on a ship, then decommission the ship to get rid of the population.  My experiments show this doesn't work.  The population just magically appears at your nearest colonized planet.  It could be a core world or a colony.  It's possible to now have a colony with more than one population.  Since the population of a colony doesn't normally increase, this could still be a good way to relocate races you don't want, but I don't know if that gives any type of bonus or any type of problem to the colony.

The creatures in space may help with your problem.  I'm not completely sure about everything that can happen.  Precursor sentinels can capture a colony ship, but they won't do anything with it.  Some other race can now capture the colony ship and use it, so it will be their problem.  I believe the same is true with pirates.  Space monsters will destroy the ship and use the debris to make a new space monster.  The former passenger will no longer exist.

I think constructors and transports may be the same as colony ships for precursor sentinels and pirates.  They are definitely the same for space monsters, which will destroy any ship and make a new space monster from its debris.

Using the creatures in space will probably be more trouble than doing something else.

It is probably best to do what the others have suggested.

This is late enough on the largest map that there are no more space monsters or pirates (or even anomalies except for the non-working ones that are heavily defended.)  I didn't know you could send your own colony ship there with another race.

Right now I have the Mimots all imprisoned. I'll try to send a colony ship (or transport ship or constructor) with other colonists (This planet had 4 mimots). I think if you place the ship in orbit, when you relaunch it you have the option to change crew.  Then just send the ship into enemy territory to be shot.  I think the transport might be best.  Or just send the ship to the corner out of the way.

I may have to do the same with the Yor. I tried to manufacture more citizens but I don't get the option. I guess because I'm not playing as the Yor.

During the Beta, Mimots and Festron could suddenly show up on one of your worlds if you were trading with them. I now remember that I imprisoned them until I could make a constructor.

Big issue is if you have to fight their empire. Really just need to pull a General Order 24 / Base Delta Zero on the pests.

I figured out how to deal with these guys. The first thing I do is IMPRISON them. Then I send a colony ship to the planet with any other race. Then I put Mimots and the Egg Layerst on constructors so they will at least be of use.  I think they won't infect my planets from there.

I don't know of any negatives of having Yor citizens except one. You can have an trait where you have unhappiness based on alien citizens.  So I keep my Yor, but still send a colony ship to seed the planet with, well in my case Humans.  With the Yor I can also use them later on for troops on my transports.

Do not feed them after midnight or give them baths either.  

I don't usually consider Mimot citizens a problem. They help grow your population, if present on your core worlds. Their growth stats (or whatever its called) is higher than other organics. I find food fairly easy to grow, so feeding Mimots is not that hard for me.

Its the Yor and minor citizens I have a problem with. They have no growth stat so population don't grow if they're the only citizens present. You have to import other types of citizens to get population growth started.

I export Festron using constructors. I often need more constructors. This stops their growth and murderous habits.

There is no good way to exterminate population or make them go away. I tried destroying colony ships back in the beta and it didn't work. The population is transferred to the nearest world that has room for them. I know of only 2 ways to get rid of population: have no worlds with room for them when you destroy a ship, or have the ships destroyed by a hostile force (doesn't work for colony ships as they can be captured; try constructors or transports instead.).