although DNN 5 will fix this, you can setup a different login using the ucanuse user module. You have to buy it and then configure it so that no one can unregister.
I believe there are instructions in the Catalook module how to setup ucanuse with Catalook
Hello,
You could always just go into the ascx for the user profile and find the unregister control then mark it as hidden by using Visible="False". That will keep everything intact so you don't get ID problems and the user won't be able to unregister themselves. Otherwise, there are a few other ways to do this, so email me if you would like me to go through them.