I have the same situation you do. My 2 basement windows are below a
deck. When I had new windows put in my house, I also changed those 2
windows. I'm glad I did. Even though I have 2 other basement windows
that are unobstructed, the 2 windows under the deck can still be
opened if I need to. Plus, what I plan to do in the future is put
lattice around my deck instead of 1X6 that are there now, so I can see
in my backyard from the basement window to look for the kids.
The replacement windows are cheap enough, probably easier than to block
the windows. Also you have a means of egress in case of an emergency.