Here's the official answer:
There are special cases where you might want to destroy all evidence of a file or commit. (Perhaps somebody accidentally committed a confidential document.) This isn't so easy, because Subversion is deliberately designed to never lose information. Revisions are immutable trees which build upon one another. Removing a revision from history would cause a domino effect, creating chaos in all subsequent revisions and possibly invalidating all working copies.
The project has plans, however, to someday implement an svnadmin obliterate command which would accomplish the task of permanently deleting information. (See issue 516.)
In the meantime, your only recourse is to svnadmin dump your repository, then pipe the dumpfile through svndumpfilter (excluding the bad path) into an svnadmin load command. See chapter 5 of the Subversion book for details about this.
Found here: http://subversion.tigris.org/faq.html#removal
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