How can I fix this script so a character won't move?

There are multiple steps you can take! The easiest way would to immediately anchor the players character upon spawning:

game.Players.PlayerAdded:connect(function(p)
p.CharacterAdded:connect(function(c)
for _,v in pairs(c:GetChildren()) do
if v:IsA("BasePart") then v.Anchored = true end
end
end)
end)

If you want to take a more complex way, you can try using the ControllerService; Here's the wiki link to it:

http://wiki.roblox.com/index.php?title=Controlling_a_Player%27s_Character

Or even a more simple way:

game.Players.PlayerAdded:connect(function(p)
p.CharacterAdded:connect(function(c)
local human = c.Humanoid or c:WaitForChild("Humanoid")
human.WalkSpeed = 0
end)
end)