The security industry. A proper regulatory regime requires mandatory licensing for anyone working in the security industry, with a history of violent crime or extremism being automatic grounds for refusal of a license. You won't catch everyone, but it isn't at all difficult to weed out the obvious thugs and prevent a security industry from being "rife" or "filled" with neo-nazis.
As if some country has solved this? In every country I know the security industries are filled with extreme right wingers and/or hooligans.