On path errors like this, I highly recommend using the command-line SVN client instead of a graphical client like Tortoise. You can use the 'svn list' command to get exact file/folder names as the repository sees them and then copy/paste them into a 'svn delete' command. That way, you'll minimize the chances of running into case-related problems.
Based on your comment to msorens' answer, it looks like the file in question does not exist in the current head of the repository. If so, there's not much you can do. Subversion treats the repository history as a single, linear series of commits. All modifications are done to the head of the repository; you generally can't go "back in time" and alter a past repository state. It's possible (but not easy) to remove a commit, but it throws off everything that comes after it. Therefore, it's only recommended that it be done to the most recent commit to the repository (to minimize the side-effects). Some other version control systems (git, for instance) have additional capabilities with regards to altering repository history. Subversion, unfortunately, does not.