I find it:
1. koji call resetBuild             # reset failed build
2. koji call createEmptyBuild  # change state
3. koji import or koji build again

But I don't know how to “Completely”  delete a build from koji and database.
Anybody knows ?

2011/4/2 Eric Zhong <ericiszhongwenjia@gmail.com>
I use " koji call resetBuild"  to clean a failed build , then i import the SRPM&RPMS directly.
SRPM be imported, and when import  RPMS, it   gives    "Error importing : Build is CANCELED"
Cant I reuse a failed NVR ?