Well, you could use very hot water with an organic disinfectant, or you could put it in the oven at a low setting, cover it with foil and leave it for 10 minutes. Be sure to check on it though lol.
I don't think using a bleach solution would be very good since the wood could just absorb it and release fumes when it's placed back into the warmth of the tank. However, if you do want to use a bleach solution to clean then use about 1 part bleach to 20 parts water. Scrub the area and leave it to soak for 10 minutes. Then remove it, and place it into hot, soapy water. Leave it for another 10 minutes, rinse it, and leave it to dry. I'm not sure what the exact effects are on the log over time, but it's certainly a thorough method. I'd personally only do all this if your snake had literally gone to town in soiling the log, if it's just a small patch, using an organic disinfectant with hot water should be fine.
Another good technique is to use white vinegar mixed in with water, it's natural and is also a great solution for cleaning the glass of your tank.
I hope that helps!
David