How to efficiently mark mail as spam before reviewing it in Mac OS X Mail.app?
One solution would be select all those messages and mark them as Junk using an iOS device, which would not load/preview emails until they’re opened.

You can also run the following AppleScript:
tell application "Mail" to set junk mail status of (messages of mailbox "QUARANTINE" of account "NAME" whose junk mail status is false) to true

Replace NAME with the name of your account, as defined under Mail → Preferences → Accounts.

You can then run that script anytime you want to mark all those messages as junk.

You could even setup a rule under Mail → Preferences → Rules:


If any of the following conditions are met: Every Message
Perform the following actions: Run AppleScript
In the dropdown click Open in Finder.
Drop your .scpt file into the window that opens up.
Select that script in the action’s dropdown menu.
Set this rule as your topmost rule, so that it runs before any rule that ends with Stop evaluating rules.