With Master Control and/or Story Progression. Don't know exact option or if is there a way to cure all Sims simultaneously (though I do know you should ought to try Nrass master control on a city hall, then go under Sim option, find a option that relates to Life State, select human and check All Sims that are within the town), but try out messing around with the options and find which one does the job without saving the game. Once you figured that out, reload the game and try the discovered method. I'm sure there are other mods out there that completely eliminate supernaturalism from all Sims.
If you oppose that and like doing things through vanilla means, than you'd have to manually cure them somewhat or through cheats. I also believe it's also can be done through
SimPE until everybody is changed to mortal without having to enter the game.
P.S. Sorry for my bad english.