Hide the back end?

Hi,
I’ve tried a variety of plugins all promising to do what I need. No luck so far.

Users can make an account with my site inorder to post stuff etc, and the theme has its own ‘user profile’ page and Log in etc… How can i stop account holders that aren’t admin’s from accessing the /wp-admin/ and dashboard stuff? It shows them with a dashboard ‘profile’ and access to the Media library.

I want to disable their access. How can i do this??
Please help!
Thanks!!