Allowing People To Create Websites And Hiding Certain Aspects

Hello, ive got Multisite set up on my WordPress blog and im wanting to allow people to create a account and create there own website on my server. I think ive pretty much figured that much out, however im wanting to hide certain aspects of the dashboard from them.

I have my own sites within the Network, and the themes from my sites im wanting to hide from people setting up a site, the same goes for certain plugins. Is there a way to hide selected themes and plugins and make them unusable by people who create a site on my network?

Also, im wanting to make the websites ad-supported. Is there a way to add adverts to the site which users cannot take down? For example, lock certain widgets and make them so they cannot be removed?

Any help would be appreciated.
Thanks.