How to disable one radio button within a group?

Hello, I am trying to figure out if you can set one radio button within a group to read only. I know how to make it work for disabling the whole radio button group with the following Mouse Up JavaScript:

this.getField("Style").readonly=true;

Within the "Style" group I have two radio buttons, "A" and "B". How do I alter the above script to just set radio button "A" to read only?

Thank you.