[Hyperlane only]Had covered all chokepoints, one colony ship slipped through - What is can I do?

I am playing a Hyperlane game only. I have built colonies and frontier posts at choke points to grab a very big area for myself. Now this went very well as I had no neighbours and successfully grabbed a big part of the map for myself. However, one motherfucker, somehow got map control over on of my systems, that was supposed to be in my borders. I was much closer with a bigger planet. And I have researched 2 techs to give me 25% more border range. In any case before I manged to get the system back the fuckers managed to colonise a system inside my area.

Here you have a picture of it: http://i.imgur.com/Kf9r8z1.jpg

I don't know why Antares don't manage to get more border control. My planet has a bigger population than both of those planets that are closest, and I am closer to the Rigel system, yet I didn't get control over it?

In any case I have to ask what to do now, because the game has given me zero info on it. If I understand it correctly I have a few choices.

1. I take control over the planet. But I don't want to do this because I don't wanna waste filling up my planet slots and sector slots because apperantly I can't have more then a few sectors. Also it's a shit system with shit neighbouring systems. So I'd rather pass.

2. Free the planet, this makes the inhabitants into a new faction but I don't want this since I want to be alone in my area.

3. Purge, I take control over the planet then purge it. However I can't enable the policy to do so because my race is invested research and military and not whatever is needed to purge the planet. I find this fucking stupid because the race I am fucked no matter what and purging is the only good option that I know of.