The only solution (and it is not necessarily an easy one) barring expensive
refrigeration units ....

Very, very, very expensive refrigeration units. For any pond of any reasonable
size, no mortal can dream of affording such a thing. The prices would be well
into the thousands $$.

I did see an interesting home brew project where the guy built a heat exchanger
deep, deep into the earth. He had a backhoe one day, and some extra time, so
just went with it. This can work very well, and your only expense is the tubing
and the pump. He was happy with it, but discovered later that he should have
gone deeper. His area (AZ) has some pretty hot ground in summer, I guess.

At a guess, doing something like building a heat exchanger underneath a
shaded patios slab would be about optimal.

Other than that, the only other solution I can think of is to buy a
refrigeration unit (and like I said, they are expensive):

Force additional evaporation, and the pond will cool some. Course, this means
replacing the water. Which might be a plus is street water is cooler than the
pond. And you have a way of getting the chlorine out quickly enough.

No, heavy shading and making it nice and deep are about all you can do here.

I missed the original message. However, if this is to protect koi, I believe
it is proper to do temperature measurements across the depth of the pond. If
the koi can escape to a moderate temperature on the bottom of the pond, I
think they'll be okay. You might want to check with a local koi specialty
place about that.

IMPORTANT POINT: high water temp reduces the ability of the water to hold
oxygen. Physical oxygenation techniques (bottom water pumping or an airstone)
are called for in this situation.

C//