Is there an independent device that requires a key to lock computers?

I have seen one in the past either online or in a catalog I can't remember. I am having problems with one of my daughters signing up for these so called "free" survey sites and we have gotten several e-mails from companys saying that they are going to be charging us on our phone bill. I have e-mailed these companies back and told them that my minor daughter is doing this. She lives with her mother who is blind and cannot see what she is logged into I am thinking that this may help solve the problem if she does not have unlimited access to the computer whenever she wants