You can always label an issue, even if it's removed from all screens and all configurations

Even if you remove Labels from all screens
and hide it in all field configurations

you can still type the L shortcut
and the Label Editor will pop up
and let you add labels to the issue.

And the bigger problem:
If you have hidden the Labels system field
and added a labels custom field
then when you press L
you still edit the Labels system field
and not the custom field.

The same thing happens when editing labels in the IssueNavigator

In this screenshot from a clean, new Jira 4.2.3 install
note that Labels is hidden and not listed on any screens

Yet in this screenshot see that I am able to label an issue
that should not have the labels field visible or editable

and in this screenshot see that I should only be editing my Tags custom labels field
but instead pressing L opens the labels editor for the Labels system field.

For us this is a big problem
because for many years
since Jira 3.10
we've been using the labels plugin.
We have had a labels-type custom field called custom tags
and there are now hundreds of thousands of label data
on hundreds of thousands of issues
for that custom field.

Now as we upgraded to Jira 4.2.3 we'd like to suppress the Labels system field
and continue using our Custom Tags custom labels field.
However, the Labels system field just won't go away
(which reminds me of an earlier suggestion I made about Jira: get rid of system fields. Instead, we're getting more system fields...).