How do we stop Koi fish in our pond from multiplying.  We put in 6 Koi
last year and we have at least 30 5"  Koi and 30 1" Koi in our 70 x
100 ft. pond.
Thank you.

I am not a Google Answers Researcher.

Might I suggest removing the females? 
According to this: http://www.geocities.com/ramcichlid/Koi-GoldfishPage2.html
  Determining the sex of Koi and Goldfish isn't too easy but after you
have had them for a while you get to see the differences.  Male Koi
are more long a slender than the females and the first ray of they're
pectoral fins are a touch thicker and the pectoral fin seems to have
more of a point than the females do but this is also not easily
detected.  The males also get these little tubercules (white dots) on
the sides of their gill plates and on the first rays of their pectoal
fins.
  The female Koi, when viewed from the top and just to the side can be
seen to be plumper than the male around the anal area.

Other hints based upon the breeding information on that page may be to
make it harder for the young to survive based upon the type of food
and environmental conditions of the pond. Of course, if watching the
movie Jurassic Park is any indication, nature will find a way :).

If this helps, great! If not, that's fine, too. The GA Researchers may
have more information and helpful hints.

Why stop them breeding?

They're worth keeping.

Just sell your overstock off every few months or so.

Thank you both for comments.  Re:  Selling off overstock.  How would
we do this?  Is there a process already set up?  I.E.  Calling
breeders to come to catch our beautiful koi?

You need to contact someone local.

I would ask the local Petshops.

Try Yellow Pages.