Yes, genast means immediately, right now or something like that. now would be nu.
Kunde jag säga ''Ni måste sluta genast!'' istället? Jag tycker att det funkar med ''nu'', typ ''Ni måste sluta nu!''.
Ni måste sluta genast is perfectly good (and an accepted answer). nu is like now, it doesn't mean quite the same as genast/immediately.
I'd say no, "cease" doesn't quite work here, and if it did, it would be closer to upphöra in Swedish.