How do I remove a device from my photo stream?

I lost an iPod touch about a year ago and the person who "found" it continues to use it (in a different country) and I occasionally still see some of their photos on my photo stream. How can I "disconnect" from that device? It kind of freaks me out that if I can see their photos, then I know they can see my photos. (the device did NOT have 'find my iphone' app loaded on it)