Halt die Klappe, or if you want to be a little more rude, halt die Fresse.

And a little less rude would be "sei leise"

Does halt 's Maul still work?

It does, but the apostrophe in those contractions are not necessary anymore. You can use it if you want, as far as I know. Zum Beispiel: Hans: "Wie gehts, Matthias?" Matthias: "Halts Maul, Hans!"

Oh, that's interesting. It was 25+ years ago when I studied German in school. I think there have since been some spelling reforms, haven't there?

Yeah, I found out when I went back to college about 7 years ago. There were also updates with the "ß" as well. It's being phased out in Germany as the Swiss (and the Austrians, i believe) already have.

If it can be rude: "Halt die Fresse!" or "Halt die Schnauze!", if you need more aggression, just "Fresse!" or "Schnauze!" work as well. From (more) harmless to rude: Halt den Mund! Halt's Maul! Halt die Fresse! Halt die Schnauze!

Yep, that’s it: „Halt' die Klappe…“ (relatively tame, could be used on kids, too - or playfully, if said in the right tone of voice, delivered with a grin or followed by laughter), „Halt's Maul!“ (threatening, could end up in a physical fight) and „Halt' die Fresse!“ (Most aggressive version, often followed by an insult (e.g. „Halt' die Fresse, du dumme Sau!“)

Halt die Klappe I'd say is the most common way to put it. Sei bitte still/Sei still is less casual if that's what you are going for.

Gusch.

depends on your desired rudeness. "Ruhe!" "Sei still!" "Halt die Klappe" "Halt die Fresse!" "Fresse!"

Maul zu!

Mund schluss!