Prevent client from changing password...

[Zorb]

In a domain type envroment where you don't want the domain client users from chaning the password required by the viewer. Entering the existing password to make a change may not be so good if you've forgoten or lost the current password. But adding an additional password just increases the chances of losing a password.

[Rudi De Vos]

Only users who has admin permission can change the password...
The security is based on the permissions of the ultravnc.ini file, you can modify that if needed.

If the current domain client can change the password, he has admin rights...nothing you do can prevent a local admin to change the passwd.