This was irritating, so I wrote an AppleScript to do it, which I then put in the Entourage Script Menu Items folder. Here's the AppleScript:
tell application "Entourage"Enter the script using the Script Editor in /Applications -> AppleScript.
set junkFolder to folder "Junk" of in box folder
set selectedMessages to messages of junkFolder
repeat with theMessage in selectedMessages
set read status of theMessage to read
delete theMessage
end repeat
end tell

