Workarounds:
- Use the Lightweight render mode of the FormDecorator;
- Use RadCheckBox instead of asp:CheckBox;
- Use the following styles:
.rfdRadioChecked::before, .rfdRadioUnchecked::before,
.rfdCheckboxChecked::before, .rfdCheckboxUnchecked::before {
    content: none !important;
}

I noticed that RadFormDecorator does not handle checkboxes in a RadListBox. I had to use an item template. It would be nice if it automatically decorated them.
Thanks!