How to deal with separated or divorced parents
A booking in Parents' Evening is actually between the parent and the teacher, not the child and the teacher. This means that where you have separated or divorced parents, as long as each parent has a record in your system (which you can check by going to Data > Parents), then both parents can separately book an appointment with a teacher.
If there is a need to make sure that the parents' do not meet by accident, then an admin can book for one, or both, so that they do not end up with adjacent appointments to see the same teacher. See Administrator: How to book an appointment for a parent for more details.
If you do not want a parent to be able to log in, you will need to either remove their parent record, though if you sync with a Management System (such as SIMS, iSAMS, etc) remember that the parent will be replaced each time you sync.