Delete the fixtures on a functionnal test

Hi,

Since couple a day, I block with the delete of my fixtures in my
functionnals test. In my controller, I need test a redirect if there
are no value.

But i load my fixtures in start of my test and in a specific test I
want delete all. Only in this method, not in other method. In API, I
discover the method delete_existing_fixtures, who seem the good
practice, but I can't find how use it. I have always the information
of method not found.

So I want know if anyone has an information for delete my fixtures in
a test.

thanks