The MagickWand interface is a new high-level C API interface to ImageMagick core methods. We discourage the use of the core methods and encourage the use of this API instead. Post MagickWand questions, bug reports, and suggestions to this forum.
Use a pixel wand to set the alpha channel value to transparent. The source distribution includes wand/wandtest.c which includes an example of setting the color value of a single pixel in an image.