there are quite a few of them out there, blocking them all is almost impossible, but some of the nasty ones like golden palace casino, etc, can be blocked with via MVPS Hosts File
. just download the hosts zip as instructed by the site, and install into the appropriate location, again as instructed by the site, checking every now and then for an updated list.
Now, as far as he is concerned, you can created a limited user account, not giving him/her administrator privileges, that way he/she cannot download anything, then creating a password on all user accounts with administator privileges. including that of safe mode...just start the computer in Safe Mode
and log into the admin account, then create a password so that way it cannot be accessed by anyone but yourself.
As far as cleaning of junk, click on the link below my signature and follow through it, if any of those casino sites is adding junkware on your computer, it will aid you into removing of it, and check the ADD/REMOVE under control panel for software that you do not recognize related to the casinos.