How to carry out the script to select a radio input block a text field?

I need your help! I want to implement a radio input that when, for example, the user selects one opcion, block  tone particular text-field.

I am using it for another function the checkbox as follows:

if (this.getField("xxx").value  == "Off"){

    this.getField("yyy").readonly=false;

}   

else {

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

    this.resetForm(["yyy"]);

}

But does not fulfill the necessary function since it allows multiple selection.