If you saved any of the yeast left, you can make a 1L starter and pitch it in the keg/secondary and it will usually clean up the diacetyl. I've had good success with it. Just make sure you use the same yeast or if that's not available, a less attenuative strain. I wrote a more detailed description about how to fix diacetyl
if you're interested. It's save more than 1 beer for me.