#+begin_src nim :tangle no
echo "this block won't get tangled"
#+end_src

Below has extra spaces before ":tangle". But that should not matter.
#+begin_src nim   :tangle yes
echo "this will be tangled to tangle_no_yes.nim"
#+end_src

#+begin_src nim :tangle specified_file.nim
echo "this will be tangled to specified_file.nim"
#+end_src