How can I stop a character from jumping?

Here, I remade the script, tested and should work;

repeat wait(0)until game.Players.LocalPlayer and game.Players.LocalPlayer.Character and game.Players.LocalPlayer.Character:FindFirstChild("Humanoid")and game.Players.LocalPlayer:GetMouse() --Repeats waiting 0 seconds until LocalPlayer, LocalPlayer's Character, LocalPlayer's Character's Humanoid, and the Player's Mouse (Don't know why I added that :P )
game.Players.LocalPlayer.Character.Humanoid.Changed:connect(function(jumpy) --If something changed then (Using the Changed event)
if jumpy == "Jump"then --If Jump changed then
game.Players.LocalPlayer.Character.Humanoid.Jump = false --Jump is false
end --The end for the 'if' statement
end) --The end for the Changed event

This script has been testing and should work, hope this helped!

I reccomend using Crazyman32's anti-jump script.

http://www.roblox.com/item.aspx?setItemId=13188099&avID=283155731