Exibir lista de usuários somente ao fazer uma busca.

admin.py

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from django.contrib.auth.models import User


class UserAdmin(admin.ModelAdmin):
    list_display = ('email', 'first_name', 'last_name')
    list_filter = ('is_staff', 'is_superuser')
    search_fields = ('email',)

    def get_queryset(self, request):
        qs = super(UserAdmin, self).get_queryset(request)
        if not request.GET.get('q'):
            return qs.none()
        return qs


admin.site.unregister(User)
admin.site.register(User, UserAdmin)

Interessante :)