Define a style -- maybe name it Code -- and set its font and paragraph
formatting to what you want  (for example, Courier New 10 pt, 0 pt Space
After, etc.). In the New Style dialog, click the Format button, select
Language from the menu, and check the box for "Do not check spelling or
grammar".

Apply that style to each block of code you insert, and besides looking
right, it won't display any spelling/grammar errors. That won't affect text
with any other style.

Yes I can imagine <g>

Select the block of text and then Select Tools > Language > Set Language
from the Menu and check "Do not check "Do not check spelling or grammar" in
the Dialog.