The usage is correct, but very rare; I observe it more among non-native speakers, who generally are less intimately familiar with the farrago of special cases that constitutes so much of English usage rules in practice, including that which has "mustn't" by convention only used in the imperative mood.

What is all this? As a native English speaker, I view mustn't as a contraction of must not; other examples include cannot and can't, should not and shouldn't, and do not and don't.

So the usage of mustn't should be exactly the usage of must not because that's literally what it is.

"You must not spend time on US conspiracy forums."

"Must" is an imperative or a command. "Should" is a recommendation.

Long time after the fact, but this was my understanding of contractions also, and was my intended usage of "mustn't". English is a crazy language, especially when we all seem to have been shipped with different compilers.

I mean you're not wrong in the narrowly denotative sense, but there's a distinction to be made between the way words are defined and the way they're conventionally used. I'm describing my observation of the latter. (So are you! But you observe differently.)

I think I am unusually permissive about where I allow contractions. I'm always tickled by that E E Cummings poem that just barely pulls off a rhyme with "year" and "appear" in its last line, "...and April's where we're.".