.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 you need to remove it from the index. This can be achieved with this command.

git rm --cached <file>

If you want to remove a whole folder, you need to remove all files in it 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, it will remove the files from other developers machines on nextgit pull.