RTFM:
GNU rm, like every program that uses the getopt function to parse its
arguments, lets you use the -- option to
indicate that all following arguments are non-options. To remove a file
called `-f' in the current directory, you could type either
rm -- -f
or
rm ./-f
The Unix rm program's use of a single `-' for this purpose predates the
development of the getopt standard syntax.