How do I delete all terms from a vocabulary (bulk delete)?
A favorite method of bulk-deleting content by content type, taxonomy vocabulary, etc utilizes the http://drupal.org/project/devel module. For deleting all terms in a vocabulary:


Enable the Devel and Devel Generate modules
Go to Development > Generate Terms
Select the Vocabulary from which you wish to delete terms
Enter "0" for number of Terms to generate
Check "Delete existing terms before generating new ones"
Click Submit


Voila-- empty vocabulary, otherwise untouched.