Can form submissions dedupe and "fill" instead of "override"?
There is a partial solution available called the No overwrite extension which will prevent users from changing their name. But this only works if they are logged into your website already.



For non-logged in (anonymous) users, CiviCRM has to "guess" who it is by using its dedupe rules. You can configure which rule the event page uses here: 



However there is no solution I know of to filter the information submitted - it will overwrite existing records if CiviCRM finds a match.