Deepfreeze is only a active backup program. It returns the computer to the stage wen deepfreeze is installed, clean and without viruses. If I disable any option for downloading and installing programs, people will complain about this, because I can't (don't) install all programs that people can use, for example, not everyone uses yahoo messenger, most people use msn messenger, so I don't install yahoo messenger on every kiosk. Sometimes people come that want to use yahoo messenger, so they download it, install it and use it. After a restart yahoo messenger is gone then because of deepfreeze, anyway this is just a example, like yahoo messenger there are dozens of programs that people can use that I don't know, by letting the option open for them to download and install any program I give them the chance to use my kiosks.
I don't know how we can fix this issue.
Anyway, for now the kid will not come back any soon

the owner of the cafe told me that he almost harrassed the kid so he's sure he won't use the method again.

offcourse this is not the good way to solve the problem
