hi, I have no idea if this will help but to control authentication of svn using the svn protocol (svn+ssh is svn protocol over ssh) then you need to add / modify the authz, passwd, and svnserve.conf files of the repository (stored in 'reponame'/conf directory).
This will let you create login credentials for users who are or are not allowed to access the repo. So whoever can connect to your server with ssh might not have the means necessary to check code out of svn.
These are the files I have they set the repo to access only by the authorised users detailed in the passwd file.
anon-access = none
auth-access = write
password-db = passwd
### This file is an example password file for svnserve.
### Its format is similar to that of svnserve.conf. As shown in the
### example below it contains one section labelled [users].
### The name and password for each user follow, one account per line.
sally = sallysPassword
Hope that was what you were looking for.