If you need to wipe all traces for some objects in your svn repository, you can do something like this:
# svnadmin dump REPOS_PATH | svndumpfilter \
<include|exclude> $string | \
svnadmin load REPOS_PATH_NEW
# mv REPOS_PATH REPOS_PATH_backup && REPOS_PATH_NEW \
REPOS_PATH
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This does not always work though, as you can read here:
http://blogs.quintor.nl/bbottema/2008/03/01/subversion-obliterate-the-forgotten-feature/