Use the regular old hydrogen peroxide you get at the drugstore, undiluted.. Just 4 or 5 drops, let it sit for a few minutes and then either tip the plant over so it can run out or stick a rolled up corner of a paper towel down in there to absorb it.
I use paper towel in a similar way to just remove water I've acidentally got into the crown. It happens quite often, but I tip it out then dab with paper towel to absorbe the moisture. I also try and water in the morning so that any I can't get with the paper towel will evaporate.