How do you freeze the players character?

You aren't advanced if you don't know string.sub.

Just anchor a character's torso.

Or you can just set the character's WalkSpeed to 0.

Kohl's Admin Command's :Freeze me Script:

if msg:lower():sub(1,7) == "freeze " then
local plrz = GetPlr(plr, msg:lower():sub(8))
for i, v in pairs(plrz) do
coroutine.resume(coroutine.create(function()
if v and v.Character and v.Character:findFirstChild("Humanoid") then 
for a, obj in pairs(v.Character:children()) do 
if obj:IsA("BasePart") then obj.Anchored = true end v.Character.Humanoid.WalkSpeed = 0
end
end
end))
end
end

This is how kohl's admin script works, (note: copying this script will not work)