With JQuery it can be done like this.
$("radio button").change(function()
{
$("textbox").attr("diabled", "disabled");
});
Or this to apply to all radio buttons
$("radio buttons")each(function()
{
$(this).change(function()
{
$("textbox").attr("diabled", "disabled");
});
});
Obviously you need to supply the proper selectors for the elements