"Obliberating" contents from a svn repository, i.e. wiping this contents from the disc, can be done as described in this article http://www.limilabs.com/blog/how-to-permanently-remove-svn-folder
It requires access to the server side svn repository, thus you must have some admin privileges.
It works by (a) dumping the repository content into a file, (b) excluding some contents and (c) wiping and re-creating the plain repository again and eventually by (d) loading the filtered repository contents:
svnadmin dump "path/to/svnrepo" > svnrepo.txt // (a)
svndumpfilter exclude "my/folder" < svnrepo.txt > filtered.txt // (b)
rm -rf "path/to/svnrepo" && svnadmin create "path/to/svnrepo" // (c)
svnadmin load "path/to/svnrepo" < filtered.txt // (d)
The repository counter is unchanged by this operations. However, your repository is now "missing" all those revision numbers used to create that contents you removed in step (b).
Subversion 1.7.5 appears to handle this "missing" revisions pretty well. Using "svn ls -r $missing" for example, reports the very same as "svn ls -r $(( missing - 1))".
Contrary to this, my (pretty old) VIEWVC reports "no contents" when querying a "missing" revision.