The default repository cannot be set to the individual user on the Web Client, since the browser does not know who's logging in yet until the user selects a repository and enters their username and password. So there's no way Web Client can determine which repository to display or log into until the user identifies themselves.
In the Web Client Configuration page, you can specify which one of the repositories need to be set as default, but this applies to all users.
The best option would be to save a shortcut in the user's browser as follows, where the repository name is added to the URL:
If using the Windows Client, you can right click on the Repositories list and remove the option for "Scan Network for Repositories". Whatever repository has been logged into (Attached) previously or in the future will be saved in the repositories list and this might help clean up the options that the user has.