Etiquette is the best form of self defence.
I have just started aikido(and posting on this bb) but I have noticed these problems also. I don't know if the below system is common in aikido since I've only been going a little while but I thought it would be interesting.
My sensei (when the class is fairly small) gets one nage to stand in the middle of the mats and everyone else to line up at the edge of the mat. The poeple lined up are all uke and they attack one after the other returning to the end of the line after being thrown or whatever. After all the ukes have had a go the first uke bows to nage and becomes nage. The old nage goes to the end of the line of ukes.
This way everyone is uke to everyone and nage is constantly getting a new uke. It stops alot of chatting because it is hard to maintain a conversation when you are required to be uke!