We all the deal, relationship maintenance is the responsibility of the application.
If there is a relationship from Employee to ParkingSpace we have to do

employee.setParkingSpace(null);
and
parkingSpace.setEmployee(null);

before doing:

em.remove(employee);

My question is, is it not better to do this using the @PreRemove listener annotation? for example

@Entity
Employee {

@PreRemove
public void preRemove() {
if (parkingSpace != null) {
parkingSpace.setEmployee(null);
parkingSpace = null;
}
}
}

and

@Entity
ParkingSpace {

@PreRemove
public void preRemove() {
if (employee!= null) {
employee.setParkingSpace(null);
employee= null;
}
}
}

It's better to do it either manually or having a helper method in your object. The reason for that is consistency between managed and detached objects. Both should be handled the same way

sorry, I am confused, you can't remove detached objects so why would consistency be an issue?
Also, when you say "having a helper method in your object" do you mean not to use the @PreRemove annotation at all, then?

Because a user used to the remove side effect, might expect that association reset is somewhat handled, which is wrong. But do as it pleases you :)