Robots go into a Faraday cage, where they must operate without any external radio control. Left to fend for themselves with just their AI wits, they fight to the death.
It used to be glorious, a free-for-all event organized by the Arachnitative Anarchs in the off-season before the Burning Man pre-show Augmented-dude capture-the-flag game. It would take place in empty parks and football fields and demolition dumps.
Then Ilay Thork threw a lot of money around, and set it up in a fancy underground installation, beneath a rebuilt Mission-district park.
An arena suitable for a Bond villain.