Stop earth rotation around its axis
Slowing the planet would have major consequences. Stopping the planet would have major consequences (here are some: Effect on sea level if the Earth stopped rotating).

Doing either of these things too quickly would have even greater consequences, but trying to predict these consequences and the effects for life on earth would be (a) a major primary research project, and (b) perhaps impossible. The answer is that we just don't know, and I doubt that anybody is about to fund the research ;-)

How to do it is, as I suspect you realise, also a very big question. I mean, if for some reason you wanted to. The difficulty is conservation of angular momentum: for the planet to have stopped turning, you need to have shifted that angular momentum somewhere else. That probably means firing stuff into space, or using tidal mechanisms (as somebody noted re the Moon), or maybe building a giant / massive flywheel on the Earth's axis and spinning it really fast the other way..... all of these are, of course, well into the realm of science fiction.