To delete material queries:
Select Delete
Material Query 
from the toolbar or the Query
menu.
Select the material queries you would like to delete.
When you are finished selecting, press Enter or right-click and select Done Selection, and the selected queries will be deleted.
Right Click Shortcut
A shortcut for deleting a single query, is to right-click ON a query and select Delete Query from the popup menu. This will delete the query you selected.
Left Click Shortcut
You can also delete a query by left clicking on a query and selecting the Delete key on the keyboard.