The small problem with your question is that
<option> is not a field attribute but an element in XML, so you cannot set or remove that using methods for field attributes.
You can remove however these elements using PHP DOM extension's methods like:
// $myradioForm = new Form('MyRadioForm'); // this is just a Joomla Form object. You use your own Form in your code like $form for example.
// if ($someoption == 1)
$doc = new DOMDocument;
// load your xml file from a path
$doc->load($file); // example: $file = JPATH_ADMINISTRATOR . '/components/com_mycomponent/models/forms/myradio.xml'
$myFormDoc = $doc->documentElement;
// we retrieve the element and remove it from the form - could be item(1) or item(2) too
$myFieldToRemove = $myFormDoc->getElementsByTagName('option')->item(0);
$parent = $myFieldToRemove->parentNode; // we define the parent node
$parent->removeChild($myFieldToRemove); // removing the element
$fieldsNew = $doc->saveXML(); // $fieldsnew here is the form without the removed option
$myradioForm->load($fieldsNew); // you load the form again and it is now without the removed option
echo $myradioForm->renderFieldset('radiogroup'); // change 'radiogroup' to whatever your fieldset name is
I'm sorry for the lot of comments in the code but this way I hope that you can understand it better. I hope you can follow and use the above (it is tried and works).
As @mick pointed out well, the above snippet cannot be applied to the case where there are many radio fields are in the Form, so you have to select the target field more precisely. Thus you have to use the
getElementsByTagName() method like below if you have more radio fields as it is usually the case in these forms:
$myFieldToRemove = $myFormDoc->getElementsByTagName('field')->item(0)->firstChild->nextSibling;
item(0) is the radio field in the Form, but it could be any selection.
firstChild->nextSibling is the first option element in the selected radio field and
lastChild->previousSibling is the third option in the field, or
$myRadioField = $myFormDoc->getElementsByTagName('field')->item(1);
$optionToDelete = $myRadioField->childNodes->item(3); // as the second option in the field.
so you can target any options you would like to... I do not consider this as the main part of this question since it is now more about learning how to use PHP DOM.
$someoptioncome from? Will showon not work for your project? May we learn more about the context of your question?