|This can be done with a script (see below).
Once written you can assign the script to a button on your toolbar.
Before you do this perhaps consider using Poco's ability to filter based on the contents of the message body.
To do this you must filter after downloading the message.
For more information on junk mail filtering see the "Junk Mail" section in the FAQ.
Create a new script and enter the following commands: (Warning, this is aircode (untested)).
ReadHeader $from "From:" %message
Extract EMail $from
ReadHeader $subject "Subject:" %message
CharCount #len $subject
ChopString $subject #len 2
AddStrings $junkfile $mailpath "junksender.txt"
SaveBody $EMail $junkfile
Set $junkfile $mailpath
AddStrings $junkfile "junksubject.txt"
SaveBody $junkfile $subject