When usenet was designed it was reasonable to expect none of the users would attack the infrastructure. Cancels were supposed to be used if someone posted something wrong and wanted to cancel their own post
The cancel feature is now used to automatically cancel the vast majority of usenet spam. Because of this many usenet spammers want to persuade newsserver administrators to disable cancels on their servers (so more people will read their spam)
One spammer, Mike Enlow, with the connivance of win.net, has released a program that allows people to issue massive cancels for messages containing any particular word. His hopes of persuading news admins to turn off cancels look unlikely to succeed, thanks to the efforts of some members of S.P.U.T.U.M.
Before checking for cancels you need to enter your newsserver in Edit → Options → News
Select Tools → Check cancels...
To check a single group, e.g. alt.stagecraft, enter *alt.stagecraft*
Each check will take some time, and will take the same amount of time for one group as for multiple groups so it makes sense to check multiple groups at once. You can specify multiple groups using wildmat format
This function will only work if your newserver accepts the XPAT command