.gitignore will prevent untracked files from being added (without an add -f) to the set of files tracked by Git. However, Git will continue to track any files that are already being tracked.
To stop tracking a file, we must remove it from the index:
git rm --cached <file>
To remove a folder and all files in the folder recursively:
git rm -r --cached <folder>
The removal of the file from the head revision will happen on the next commit.
WARNING: While this will not remove the physical file from your local machine, it will remove the files from other developers' machines on their next git pull.