how do you Lock a Player's camera?

I have made numerous 2-Dimensional games on ROBLOX and this is usually the script I use to set a 2-Dimensional environment for the player.

function GetMass(object)
    local mass = 0
    if pcall(function() return object:GetMass() end) then
        mass = object:GetMass()
    end
    for _,child in pairs(object:GetChildren()) do
        mass = mass + GetMass(child)
    end
    return mass
end

function onPlayerRespawned(newPlayer)
    wait()
    local torso = newPlayer.Character.Torso 
    local bp = Instance.new("BodyPosition")
    bp.Name = "TwoD"
    bp.position = torso.Position
    bp.P = 1000000
    bp.D = 1000
    bp.maxForce = Vector3.new(0, 0, 1000000)
    bp.Parent = torso
    local bf = Instance.new("BodyForce")
    bf.force = Vector3.new(0, 100*GetMass(newPlayer.Character), 0)
    bf.Parent = torso
    newPlayer.Character.Humanoid.WalkSpeed = 24
    script.LocalScript:Clone().Parent = torso.Parent
    newPlayer.Character.DescendantAdded:connect(function()
        bf.force = Vector3.new(0, 100*GetMass(newPlayer.Character), 0)
    end)
    newPlayer.Character.DescendantRemoving:connect(function()
        bf.force = Vector3.new(0, 100*GetMass(newPlayer.Character), 0)
    end)
end

function onPlayerEntered(newPlayer)
    if newPlayer.Character then
        onPlayerRespawned(newPlayer)
    end
    newPlayer.Changed:connect(function (property)
        if (property == "Character") and newPlayer.Character then
            onPlayerRespawned(newPlayer)
        end
    end)
end

game.Players.PlayerAdded:connect(onPlayerEntered)

So, you can put this script in the Workspace. Then insert a LocalScript inside the script (yes, that is script-ception) and paste this into the LocalScript.

lockCamera = true --set true if you want the player to move the camera on a vertical axis only and zoom in and out as much as they want, if you want the player to have a complete locked camera keep it as true
    distance = 15 -- you can modify this to your liking
    height = 5 -- you can modify this to your liking

local torso = script.Parent.Torso
local center = Instance.new("Part")
center.Name = script.Parent.Name .. " Center"
center.Transparency = 1
center.CanCollide = false
center.Size = Vector3.new(1,1,1)
center.Position = torso.Position
center.CFrame = CFrame.new(Vector3.new(0,0,0),Vector3.new(0,0,-1))
center.Parent = game.Workspace
local bp = Instance.new("BodyPosition")
bp.position = center.Position
bp.maxForce = Vector3.new(1000000, 1000000, 1000000)
bp.Parent = center
local bg = Instance.new("BodyGyro")
bg.maxTorque = Vector3.new(9e+005, 9e+005, 9e+005)
bg.cframe = center.CFrame
bg.Parent = center
local cam = game.Workspace.CurrentCamera
cam.CameraSubject = center
cam.CameraType = Enum.CameraType.Attach

while torso.Parent do
    wait()
    center.BodyPosition.position = torso.Position
    if lockCamera then
        cam.CoordinateFrame = CFrame.new(Vector3.new(center.Position.x + distance,center.Position.y + height,center.Position.z))
    end
end

center:Remove()

Hope this helps!

You could try setting the camera subject to the humanoid within the dummy model. That is how ROBLOX has the default camera subject set as.