How would I stop players from rotating their character with mouse or keys?

If you wish to only edit camera movement, replace the default CameraScript with the following code (in other words, insert the following into a LocalScript named CameraScript parented under the StarterPlayerScripts folder):

RunService = game:GetService("RunService")
player = game.Players.LocalPlayer
camera = workspace.CurrentCamera
character = nil

rotation = Vector3.new() --set the defualt camera rotation (must be in degrees)
offset = Vector3.new() --set the distance the camera should be from the players head

player.CharacterAdded:connect(function(char)
    character = char
end)
RunService:BindToRenderStep("Camera", 200, function()
    if character and character.Head then
        camera.CFrame = character.Head.CFrame+CFrame.new(offset)*CFrame.Angles(Vector3.new(math.rad(rotation.X), math.rad(rotation.Y), math.rad(rotation.Z)))
    end
end)

I didn't test the script, but it should work.

game:GetService("ContextActionService"):UnbindAllActions()