How can I stop the cooking in my chicken and noodle soup?
rfusca already gave very good suggestions for the literal question from the title. However, you can also address your problem the other way round. 

First, cook the soup until your noodles are al dente (but will become just right while cooling at a normal speed). Take the big pot of soup off the heat. 

Second, take a small pot, and fill it with just one portion of soup per eater. Put it on the heat, and cook until the noodles are done. (Alternatively, put the single portions into porcelain bowls and microwave until the noodles are ready - it spares you washing an additional pot, but I wouldn't nuke a good soup for no reason). 

Third, eat your cooked soup portions and let the big pot of soup slowly cool on its own. 

Fourth, freeze the soup from the big pot. Finis.  

Note that from a food safety point of view, you are better off with flash cooling the soup. But what I outlined here is probably much simple and hassle-free. Plus, 2-3 liters of soup minus a portion or two should spend less than the magical 4 hours in the danger zone while cooling. If you are doing this with a very big pot of soup and feeding lots of people, you should probably prefer a rapid cooling.