Sounds like an illiquid option, if there are actually some bidders, market makers, then sell the option at market price (market sell order). If there are not market makers then place a really low limit sell order so that you can sit at the ask in the order book. A lot of time there is off-book liquidity, so there may be a party looking for buy liquidity.
You can also exercise the option to book the loss (immediately selling the shares when they get delivered to you), if this is an American style option. But if the option is worthless then it is probably significantly underwater, and you'd end up losing a lot more as you'd buy the stock at the strike price but only be able to sell at its current market value. The loss could also be increased further if there are even MORE liquidity issues in the stock.