Have you tried "End Hanging Disconnect"?
"End Hanging Disconnect is a Mac OS X Applescript application that will force-quit the infamous "endless disconnect" that can sometimes occur on dialup Internet connections. Simply run the application and enter your (admin) password. You may then reconnect to the Internet without having to restart the computer.
This <new> version repeats the killall command 5 times (with 5 second delay in between tries), since the first try frequently does not end the pppd processes. If the command is successful on the 1st, 2nd, 3rd or 4th try, a subsequent dialog will indicate no matching processes were found. Just dismiss the dialog as needed; no harm done."