I don't get all that much spam, and as a result I leave things in
training mode, so I can't just drag to the junk mail folder.
Besides disabling the preview pane in some way, another way to
get around this problem of not wanting to view the email is to
highlight a piece of non-spam mail, then cmd-click on the soon-
to-be-junk mail (so now you have two selected). Then click the
"set to junk mail button". Now select the non-spam by itself,
and take away its junk mail label. This leaves you with the
spam designated as junk mail, and it was never viewed since the
preview pane displays nothing for multiple email selections.

Also, if you have multiple spams to get rid of at once, you can
select one piece of non-junk and then all the other spams
together and it won't take any longer than doing it with one
piece of spam.

Just throwing that out there as an alternative.

---
\"Alcohol: the cause of-- and solution to-- all of life\'s problems.\"
--- Homer Simpson