I block amazon, period. If I detect an IP being used for proxying I issue a warning** on the reject page to the effect they should use direct IP access. This also applies to some G, Y and other proxy sources.
** that's if I already know the forward-for IP is a valid DSL line. If it's a server they are on their own; if I do not yet know its status, they are on their own until I parse it later in the day.