On the Flowmaster H-pipe I have, there's a little connection for it right on the pipe. All it does is route the air pump's flow into the exhaust stream. I'd highly suggest you not just block it off, otherwise you risk blowing the pump or the control valves. Just tie it against something and let it blow into the air, or cut a hole in the exhaust and weld the silly thing in.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 