I think people are a little confused, understandably, what happens now is that when your click on the "Accept" button, this doesn't actually accept the challenge there and then, I think Russ will probably tidy this up when he launches option C in the next few weeks.
What actually happens when you click "Accept" is that you get taken to another screen where you can see exactly who is in the challenge and alter who you want to put against them. Once you done that you can fully accept and start the challenge. This gives a big advantage to the challenged clan leader at the moment.
If you don't wish to accept the challenge once you've seen who is in the line up you cannot "decline" it on that page as there is no button to click, again this is something that Russ will probably tidy up, to decline instead you will have to click "Back" in your browser or navigate away from the page somehow and then in the first screen you can click "Decline".
This might explain why a lot of the challenges I've issued have just sat there, so I hope this clears things up.