I'm using k2. I need to restrict "group A" users can see "Items" only.
How can I do that?
Any dirty/hacky ways are acceptable no matter what.
You don't have to use any component or plugin to control your k2 access. You can create groups and set permissions in Joomla Users Groups.
There is no need to apply any hack in k2 code when solution can be built by using triggers.
If you are comfortable with coding then you can create a plugin which work on k2 triggers and restrict usergroup to access set of resource only.
K2 triggers onK2PrepareContent event which can be used in the plugin.
Add checking of the user permission in this event in your plugin and redirect if user is not allowed to access.
Another event which you can utilize is onAfterRoute event, it's a joomla event.
In this event you can check the option, view, task and layout to confirm the current access page. And apply usergroup checking.