There are couple of ways to make a read only field on qListForm. For the scenario you described, seems that using Form Component Behavior may be one way to go.
Using Form Compnent Behavior, you can define the behavior for specific field, group or tab to either disable/enable, hide/show based on condition.
The other method simply involves using Calculated Format field type and specify field format as <%Field Name%>, which will render the field as readonly field without input box next to the field name on the form.