As of 2024 this mod no longer works. I tested it. Downloaded the mod, reloaded my mods, type in /spread twice, making sure I got the message saying the biome spread is disabled. Went over to a crimson biome, and marked grass blocks. A little later I went back to the spot I had marked with floating blocks, and the crimson had spread even though the mod was enabled.

Does not work in 2024, this mod is abandoned and hasn't been updated in over a year