'carefully'.

I had a similar issue with a sonicare toothbrush. Despite being waterproof, it eventually let water in, which corroded the circuit board, and caused it to go on and off at random intervals. I broke into it (the bottom is a plug), cleaned it, but this still didn't completely resolve it. So had it apart again and cut the battery wire.

TL:DR, break into it and cut the battery wire.