How can I turn-off preview for just the spam folder

I prefer to have mail preview enabled. But that includes the spam folder as well. 
Sometimes the content of a spam email is NSFW, which complicates the processes of scanning spams for any non-spam emails.

So, how do I disable preview for just the spam folder?

I think you are referring to the message panel. It is not possible to turn this off for only specific folders.

The only way you could do it is to turn the message panel off before checking the spam folder, then turning it back on afterwards.

You will find the setting in Menu > View > Layout

I guessed that was the case.
However, it would be very convenient - speaking about this use-case - to be able to put this command (hide messages panel) as a short-cut on the window header, besides of the commands for reply, delete etc.