If you are the creator of the form you have two options to solve your problem:
The simple one is to create a reset button for the group of radio buttons, using Adobe Acrobat. Create the button and add a "Reset a form" action in its Mouse Up event, selecting just the Group of radio buttons to be unchecked.
The second and more complicated option is to use JavaScript and add the following script as a "Run a JavaScript" action to the MouseUp event for each choice in the group so you can uncheck the selected radio button:
if(event.target.lastVal == event.target.value){
event.target.lastVal = null;
event.target.value = "Off";
}
else{
event.target.lastVal = event.target.value ;
}