How to make my character stop when using this?

So i have a script here that fires my fireball. But i want to make it so it stops the players movement when they press r to fire it. So how could i go about doing that?

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
local RemoteEvent = game.Workspace.Fired
local UIS = game:GetService("UserInputService")

local Debounce = false

UIS.InputBegan:Connect(function(Input)
    if Input.KeyCode == Enum.KeyCode.R then wait(0.5)
        if Debounce == true then return end
        RemoteEvent:FireServer(Debounce)
    end
end)

You could anchor the character's torso before you perform your wait(0.5), then unanchor it afterwards.

UIS.InputBegan:Connect(function(Input)
    if Input.KeyCode == Enum.KeyCode.R then
        if Debounce == true then return end
	-- anchor torso here
	wait(0.5)
	-- unanchor torso here
        RemoteEvent:FireServer(Debounce)
    end
end)

To make the player stop moving, you would need to configure with their character.

A method would be to temporarily make the Humanoid's Walkspeed to 0 and back to normal speed.

If you don't want the player to jump, you can also set the Humanoid's JumpPower to 0 and back to its original.

This can be done in the Server Script you linked to the RemoteEvent if this is Filtering Enabled.