How do I clear a players backpack when the player touched a part?

I will assume that you know very little Lua, or Roblox Lua to be more precise.

To start off, you need to have something that the player will touch, likely a part. Position the part at the entrance, so that you know the player will touch it, and place a Script (Server Script) as a child in that part with the following code:

local detection = script.Parent -- A part that the player touches
detection.Touched:Connect(function(otherPart)
    for _,v in pairs(game:GetService("Players"):GetChildren()) do
        if otherPart:IsDescendantOf(v.Character) then
            v.Backpack:ClearAllChildren()
        end
    end
end)

Cheers. Accept this if it helped you.