How to refer the player that touched it?

I *believe * you refer to person who touched it as the parent. So in your case:

game.Workspace.PARTNAMEHERE.Parent.Head:Destroy()

Also try the FindFirstChild method. So:

arm = game.Workspace.PARTNAMEHERE.Parent:FindFirstChild("Right Arm")
arm:Destroy()

I hope this works

To refer to a name with a space, we use square brackets ([ and ]) to index instead of a dot (.), and then the string of the name:

person["Name With Space"]
-- vs
person.NameWithoutSpace

The part that touches us can be a leg or an arm or the head or torso. Those parts make up what a player's Character. Once we get the Character, we can grab the corresponding Player.

local D = Workspace.D

function onTouch(part)
    game.Workspace.Player1.Head:Destroy()
    character = part.Character
    if character then
        player = game.Players:GetPlayerFromCharacter( character )
        if player then
            -- It is a player
            -- (as opposed to some other random object hitting it)
            character.Head:Destroy()
        end
    end
end

D.Touched:connect(onTouch)