Hi Martin,
Follow the step below to disable the menu in footer :
Goto => Appearance => menu => Create menu (Say footer menu save but leave it empty). After that,
Goto=> Appearance => Customize => Header Setting => Navigation => Footer Menu (Select the empty footer menu which you have already created.) and save and publish.
Regarding to footer height, Please add the following css in child theme style sheet :
.flat_responsive_footer {
padding: 0rem 0rem;
}
Thanks