Yes, your form visitors can uncheck radio buttons. Once logged in to your 123 Form Builder account, go to the Editor of your form. Select the Single choice field and check the option Clearable, on the left.
Once the Clearable option is enabled, 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.