Can I find out who is currently using my Airport Express speakers, and kick them? If I want to stream music to an AE when someone else is connected, I get a message "The speaker XXX is currently used by someone else". 

Right now, both my flatmates are not connected to the AE but I still get that message - the AE seems to be stuck. It appears all I can do is unplug the unit and plug it in again.

Is there a way for me to kick the current user, or at least see who was last connected to it?
Change the WiFi password. If I am not wrong, you can only access AirPlay speakers via WiFi. So change it and the person in question won't be able to connect. 

If you want to find out who it is, go on to your router interface something like 192.168.X.X and see who is connected. Most people name their computers, so will probably show up. Sorry if I can't be more specific.

Also, maybe there is a bug with your AE (doubt it). Maybe you should reset it.