I have a couple of examples that are a little bit complicated. I think it has to depend on how much the person has done (creatively) means to you.
Not comic book related, but the murder-suicide of wrestler Chris Benoit is an example of a complicated situation. At the time I was able to separate the wrestler from the man (because he was a great wrestler who was playing a character on TV). If he hadn't committed suicide, however, I'm not sure I could follow him.
There are more recent examples that are bad such as Rolph Harris and, to a lesser extent, Hulk Hogan, but I wasn't really a fan of either to begin with so it's easier for me to distance myself from them pretty much forever.
More pertinently, Frank Miller is a difficult man to like for his various controversial views, but he has written some great stories over the years.
Ultimately, I think it's got to be about the artwork first and foremost. If the artwork itself is something you find offensive then that's something you can choose to stay away from, but otherwise I don't think it's unethical to enjoy the works of people who are less than savoury.
Really interesting question to bring up, though. It's definitely food for thought and something I'm going to ponder over for a while!