The easiest way is by unchecking the "auto logon" checkbox in the GUI Settings. However, if you are not able to start the client in the first place, you can remove the "messenger.properties" file in the .sunmsgr directory. This directory is located in your home directory on a Unix system (such as Solaris or Mac OS X) or in C:\WINDOWS or C:\Documents on a Windows system. If you choose not to delete the messenger.properties file but only want to remove the offending line, just delete the line that begins "net.password=."
Note: Changing of the messenger.properties file is not advisable. Hence this workaround may not work in future but it will certainly work for the current version.