Do nothing and disconnect...
or send the new BYE (0x02) descriptor to every connected peer:

http://rfc-gnutella.sourceforge.net/...ls/BYE/Bye.txt
http://groups.yahoo.com/group/the_gd...ls/BYE/Bye.txt

PS: If an user decides to disconnect while uploads are in progress, you might want to give him a warning message and suggest to stay online a few more minutes.