Well, in the video below you can see this guy came up with the idea to fit a lock on a normal computer so that people can't even turn it on.
What does it do and how does it work?
First you need a lock that can turn something on/off by turning it right or left with the key. Once you get the lock you need to connect it with the power switch on the computer, like two parallel switches. That means if you try to turn on with the one switch you can't turn it on until you turn on the other switch too. You need to turn both switch on to turn on the computer and in this case one normal switch and another with the key. So if you disconnect the power with the key switch and take the key with you then no one will be able turn the power on to use the computer. Simple as that.