That is because you have a match currently pending. To remove yourself from the ladder forfeit your upcoming match then you will be able to quit the ladder (by hitting the link that says Quit, which is right next to the Admin link) _________
Actually I continually had this problem too. I tried removing myself from the 1v1 ladder many times, whether I was being challenged or was available, but I could never do it. The screen would say I was successfully removed, but I never actually was. I was still on the ladder and could still be challenged/challenge. Finally, one day, for some extremely random and unknown reason, I was suddenly off the ladder. So thank you to whatever admin did that.