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Which version of XOOPS are you using? With 2.0.13.2 users don't have the option to change their username. They can change their real name though. With 2.2.x I can see what you are referring to, but I don't see a way of disabling this in the Extended Profiles administration. If you're running 2.2.x, you may need to hack the module to remove that field from the edit profile page.
Best Regards,
James
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.