General usage for cleaning your working directory of untracked files:

$ git clean [-d] [-f] [-i] [-n] [-q] [-e <pattern>] [-x | -X] [--] <path>...

Config setting:

• clean.requireForce

Prerequisites: gitignore

Cleans the working tree by recursively removing files that are not
under version control, starting from the current directory.

Normally, only files unknown to Git are removed, but if the -x
option is specified, ignored files are also removed. This can, for
example, be useful to remove all build products.

If any optional <path>... arguments are given, only those paths are
affected.

General execution options:

-i, --interactive
    Show what would be done and clean files interactively.

-n, --dry-run
    Don’t actually remove anything, just show what would be done.

-q, --quiet
    Be quiet, only report errors, but not the files that are
    successfully removed.

Forcing the issue:

-f, --force
    If the Git configuration variable clean.requireForce is not set
    to false, git clean will refuse to delete files or directories
    unless given -f, -n or -i. Git will refuse to delete
    directories with .git sub directory or file unless a second -f
    is given.

Removing directories as well:

-d
    Remove untracked directories in addition to untracked files. If
    an untracked directory is managed by a different Git
    repository, it is not removed by default. Use -f option twice
    if you really want to remove such a directory.