Use the command /region flag [Your region] water-flow DENY You will be able to cause water spread still because your op only normal players cannot use water spread in that area OR Use world edit I think that works too
Isn't high-frequency-flags for __global__ ? Anyway, OP: It should work, you might be doing it wrong. Once again, in-game, type: /region flag <regionname> water-flow deny. <regionname> is the name of your region. If it still flows, remove the water and place it again.