There is some powder I bought at the hardware store. Put it on the stump and it decays it. Results aren't immediate but it worked for me. Took about a season and then it was all out of there. Had to go back a few times to gradually remove the decayed wood though. If you put it on there more often than I did it probably works even quicker. Don't know if that would work well for really big stumps though.
Think it was something like this:
http://www.homedepot.com/p/Spectraci...20-4/202097353