local forbidden = {"unpack", "5"}

local contains = function(array, search)
    for index, value in pairs(array) do
        print(index, value)
        if value == search then
            return true
        end
    end
    return false
end

setfenv(1, setmetatable({print = print}, {
    __newindex = function(self, key, value)
        if contains(forbidden, value) then
            print("Sandbox: attempt to set " .. key .. " to " .. value .. ", a forbidden " .. type(value) .. "!")
            return rawset(self, key, nil)
        else
            return rawset(self, key, value)
        end
    end,
    __index = function(self, index)

    end
}))

x = "5"   --> Sandbox: attempt to set x to 5 (a forbidden value)
print(x)  --> nil