.gitignore after commit

No you cannot force a file that is already committed in the repo to be removed just because it is added to the .gitignore

You have to git rm --cached to remove the files that you don't want in the repo. ( --cached since you probably want to keep the local copy but remove from the repo. ) So if you want to remove all the exe's from your repo do

git rm --cached /\*.exe

(Note that the asterisk * is quoted from the shell - this lets git, and not the shell, expand the pathnames of files and subdirectories)

If you have not pushed the changes already:

git rm -r --cached .
git add .
git commit -m 'clear git cache'
git push