Using the "Single-Term Taxonomies" plugin you can limit the taxonomy terms chosen for a post to maximum one.
Limit the taxonomy terms chosen for a post to maximum one. The Single-Term Taxonomies plugin replaces the standard checkboxes with radio buttons. Very helpful for taxonomies (like "brand" for example) where it is not appropriate to have more than one term picked. Works for both hierarchical and non-hierarchical taxonomies and uses the WordPress stock interface.