How do I disable all fields when sending a pdf through the "submit a form" button?

If this solution needs to work in both Adobe Acrobat and the free Adobe Reader, your only option is to set the fields to read-only in your form. You can do this by running a loop over all fields and then set the "readonly" property to "true":

for (var i=0; i<this.numFields; i++) {
    this.getField(this.getNthFieldName(i)).readonly = true;
}

Depending on how your button is set up, you may have to add two different actions (or add this script to your already existing JavaScript action):