You can't force users to stay with the same username in IRC.
It's not possible, and even if it was, it isn't a good practice.
And the Java-IRC your talking about is PJIRC, not Java-IRC.
The reason why I say this is if you wanted them to keep the same username ALL THE TIME, you would have to disable /nick. Or do something like that.
If you want to make it so that users can steal usernames such as "Wizet" or "Admin" or MFHOME, etc., then you might want to use a nickserv, and force users to enter a password to use a username, or be kicked.
Otherwise, you can't get members to stay with the same username.