Don't use Simple Green... I hope you at least diluted it 10:1, but don't ever use Simple Green for anything but maybe tires.
For over the counter stuff, I'd try Mother's leather cleaner. Get the stuff in the red bottle, not the foaming, higher end stuff. It's thicker and will sit to allow it to dwell. Use a brush to lightly agitate between the grain. Go in in up and down, side to side, and circular motion to make sure you get through all of it.
If that doesn't work try 1Z Einszett Deep Plastic Cleaner. Same technique without letting it dwell.
Other than that, I'd see if it just goes away. My friend sat in my car (tan leather seats) with a pen in his back pocket and after a few regular interior cleanings it went away. I don't know what type of pen it was and I don't know if the oil based ballpoint pen ink is different than gel or whatever is used in the Uniball pens, but I wouldn't worry about it too much.
Please don't just try cleaning ut over and over and over again. The last thing you want to do is cause premature wearing for a little pen mark (if that's all it is of course).