If you had sufficient influence Mars would have flipped to your side eventuallu. At the beginning of the game your borders only extend to your colonized planets. There is a UP proposal that bans colonizing in another players zone of influence.

That planet would've flipped to you in around 10 turns or less. Silly reason to ragequit.

Destroy it if you didn't want it that much?  Sell it to other civs  (even if it isn't that valuable)?  Just because you eventually get it doesn't mean you have to keep it.

Regardless, the devs are very firm on this point: Your cultural influence is NOT your political border no matter how much psychological comfort they might provide as such.  They only  become Political Borders via a UP vote.  This ain't changing any time soon.  But if you really must have Closed Borders, mod it in.  It can't be that difficult.

I find having two planets in my home system to be invaluable for expansion purposes because you get twice as much population growth to fuel your colony ships in the early game. Even if "Mars sucks, it's too small", if it does nothing but sit there and generate colonists it's still magnificently useful.

Which only lasts 20 turns. I never allowed planets in my borders to go uncolonized because of the silly part of this game where you can colonize in other peoples territory. In the tutorial I took over all of the Iconian territory by colonizing their neighbour planet. Always thought that was a really stupid game element.

Borders are not important enough, ships and colonies can penetrate borders without penalty or consequence....wtf