I have been using KMPlayer for a long time, along with CoreAVC codec pack for amazing H.264 playback. I just recently decided that I was getting tired of the borders on my player, so I figured out how to remove them. Here are the steps.
Tired of the borders on your player? Remove them!
Launch KMPlayer, right click on it, go to Options > Preferences.
Go to General > Skins and then move the thickness border all the way to the left.
Right click, go to Skins > Control: Default (AutoHide).
Right click, go to Options > Preferences > General > Screen and then check both of the boxes under Aspect Ratio.
Your borders should now be gone! 🙂 Example of what it looks like below.