The messy way to abort a wu using BT is to run a batch file containing boinccmd.exe commands to detach then attach to the project.
Code:
C:\Program Files\BOINC\boinccmd.exe --project http://www.bitcoinutopia.net/bitcoinutopia/ detach
CHOICE /N /D Y /T 20
C:\Program Files\BOINC\boinccmd.exe --project_attach http://www.bitcoinutopia.net/bitcoinutopia/ auth
Like I said, it's messy...
Btw, CHOICE is a Windows only program. the /T 20 is a command line switch that instructs CHOICE to wait 20 seconds before continuing.