There's already no chain reaction - the control rods, plus the boron in the water they're pumping in absorb too many neutrons to sustain a reaction. The heat is not from uranium fission, but rather from the fission products made while the reactor was running undergoing spontaneous decay. This process cannot be stopped; you can only wait for it to slow down and cool to a reasonable level.
The reason they want to keep it cool is to prevent the fuel rods from melting. While melting won't immediately mean a release, it would mean the loss of two of the five barriers to a radioactive release, as well as quite a lot of heat for the remaining barriers. It would also significantly complicate later cleanup. As such, it is preferable that the fuel rods remain intact if at all possible, and this means attempting to cool them.
The reason they want to keep it cool is to prevent the fuel rods from melting. While melting won't immediately mean a release, it would mean the loss of two of the five barriers to a radioactive release, as well as quite a lot of heat for the remaining barriers. It would also significantly complicate later cleanup. As such, it is preferable that the fuel rods remain intact if at all possible, and this means attempting to cool them.