Yes, your form visitors can uncheck radio buttons. In your CaptainForm plugin, go to the Form Editor section of your form. Select the Radio Button field and check the option Clearable, on the left.
Once the form user hovers over the selected option, an X will appear next to the label. When clicked, the option is unchecked. Add field instructions to instruct your form users on how to deselect the option they have selected if needed.