I'm quite new to Subversion but from my research I understood that SVN users are basically the Linux users.
My question is how can I unbind them? Can that even be done?
It all depends on the method you use to access your repository. Basically there are three ways:
Options 1 and 3 don't require local users on the server, while option 2 does.
The Subversion handbook has a nice summary of the different server types and why to choose which one.
I personally have only used the Apache method, which allows you to create a file with usernames and password and their respective permissions, and it has worked well for us.
SVN users are equivalent to system users if you're using the svn+ssh access method. To "unbind" svn users, you'll need to move to another access method, perhaps svnserve or apache2+mod_dav.