If you want the full behavior of streams, the best is probably to use:
File.open(File::NULL, "w")
Note that File::NULL is new to Ruby 1.9.3; you can use my backports gem:
require 'backports/1.9.3/file/null' # => Won't do anything in 1.9.3+
File.open(File::NULL, "w") # => works even in Ruby 1.8.6
You could also copy the relevant code if you prefer.