Make a radio button in InfoPath form read only

Conditional formatting is the answer. To set the control to always be disabled do the following:

1. Right click on your field and select Rules
2. Create a new formatting rule and click Condition
3. Choose The expression from the dropdown 
4. type true() or 1 in the text field (this tells InfoPath to always apply this formatting)
5. Check the Disable this control checkbox.
6. Click OK