How do I exclude messages from a particular board using a LiQL query?

dustin I haven't checked the documentation but I think you can use the clause 'IN', so:

SELECT * FROM messages WHERE category.id IN ('xxxxxxxx','yyyyyyyy','zzzzzzzz')  AND depth=0 ORDER BY post_time DESC LIMIT 15

It's not the best has you might add categories in the future, or you might have a long list of categories ...

Another option is to run your request for all categories, then add a if statement to eliminate everything from your test category.

SELECT * FROM messages WHERE  depth=0 ORDER BY post_time DESC LIMIT 15

This isn't the correct freemarker syntax, just an idea:

if message.board.id != 'test' then {do what you want to do with your messages }