How do I only allow 1 category to be chosen at a time?

I have usually been able to find what I need on the forum, but how do I limit the bloggers to choose only one category for their post at a time? It seems some of the bloggers are selecting every category as a way to get their blog out to move readers but it's ruining the category structures.
Tight now in the create blog window, the category selector is multiple select. How do I change it to single select?

Thanks,

Adam