How can make Django permission_required decorator not to redirect already logged-in users to login page, but display some message

A quick and dirty solution would be to write your own decorator to do this. Something like this:

decorator_with_arguments = lambda decorator: lambda *args, **kwargs: lambda func: decorator(func, *args, **kwargs)

@decorator_with_arguments
def custom_permission_required(function, perm):
    def _function(request, *args, **kwargs):
        if request.user.has_perm(perm):
            return function(request, *args, **kwargs)
        else:
            request.user.message_set.create(message = "What are you doing here?!")
            # Return a response or redirect to referrer or some page of your choice
    return _function

You can then decorate your view thus:

@custom_permission_required('my_perm')
def my_view(request, *args, **kwargs):
    #Do stuff

Since django 1.4 permission_required has a raise_exception parameter that you can set to True to have an unauthorized PermissionDenied exception raised

Eg. to give an exemple on a Class Based View:

from django.contrib.auth.decorators import permission_required
...

class MyView(TemplateView):

    @method_decorator(permission_required('can_do_something', raise_exception=True))
    def dispatch(self, *args, **kwargs):
        return super(MyView, self).dispatch(*args, **kwargs)

Ref:permission_required decorator doc