I painted my Casio scientific calculator blue at school. I've still got it, it must be nearly 15 years old and works fine.

Get some fluorescent paint and coat as much of it as you can and paint your names into it. It'll be too ugly to steal. Or just glue a bit of string to it and tie it to the table.

Or just don't leave it lying around if you know the area is filled with theiving bastards.