Is it possible for an artist to do something so detestable that his or her work should be banned?

Wow. That's a tough one, primarily because as people we find it difficult to separate the creator from the work itself. In a perfect world, the work would be able to stand on its own merits and the creator's life wouldn't be taken into account when analyzing whether the work itself held value. I believe even a detestable person has the capacity to create something good (after all, in my belief system, we are created in the image of a creator, so creating comes naturally to us all in some way). 

For example, had Charles Manson written a great play, would it be "moral" to perform it because of the awful, horrible person he was? 

Personally, as long as the play itself wasn't detestable, I wouldn't hold it against a theater company who chose to perform it. But I'm sure the families of Manson's victims might feel differently -- and with good reason. 

In my own life, I know that Richard Wagner's symphonic works are often associated with Hitler and Wagner's own anti-Semitic views, but it doesn't make me appreciate the simple beauty of the melodies any less. 

I tend to discourage wholesale banning on any official level anyway, and I prefer to leave it up to individual people and companies to make those decisions based on their beliefs, values, and clientele. For example, a family-run, community theater might find performing a Manson-penned play a distasteful endeavor and refuse to produce it, but another theater troupe might enjoy sharing the work in spite of the Manson connection. It becomes, at least to me, a matter for the individual and individuals of the company to decide for their circle of influence, not for the governmental powers that be to decide for the rest of us. 

True censorship makes me feel very, very uncomfortable, because it involves making decisions about what's best for the whole of society, and I'm not content to let others make that decision for me -- or for me to make that decision for others, except for perhaps minors in my own house.  

(Thanks to James Wynn for this question.)