I've been trying to override the default user icon action.  

my_plugin/classes/Elgg/MyHooksOverrides/EntityIconOverride.php :  

    <?php

    namespace Elgg\MyHooksOverrides;

    class EntityIconOverride {
        public function __invoke(\Elgg\Hook $hook) {
            return false;
        }
    }

my_plugin/elgg-plugin.php :  

    <?php

    return [
        'plugin' => [
            'name' => 'My Plugin',
            'activate_on_install' => true,
        ],
        'view_extensions' => [],
        'hooks' => [
            'prepare' => [
                'entity:avatar:prepare' => [
                    \Elgg\MyHooksOverrides\EntityIconOverride::class => [],
                ],
            ],
        ],
    ];

I've also tried this setup with `entity:icon:prepare` :  

    <?php

    return [
        'plugin' => [
            'name' => 'My Plugin',
            'activate_on_install' => true,
        ],
        'view_extensions' => [],
        'hooks' => [
            'prepare' => [
                'entity:avatar:prepare' => [
                    \Elgg\MyHooksOverrides\EntityIconOverride::class => [],
                ],
            ],
        ],
    ];

However, this is not having any effect. The hook should prevent the upload from proceeding, but the upload goes on successfully meaning that the plugin hook never gets called.

How can I make sure that my plugin hook gets called / OR, in other words, how can I override the default user icon upload action?

Thank you all in advance.