I tried the usual recipe to reverse-merge an unintended commit:
svn merge -rBAD_REV:BAD_REV-1 .
and was perturbed to get, as a response, merely a message about mergeinfo. No actual revert of file content to the previous rev.
I went and did the fix up in another tree where I could use svn 1.6.x, but I ended up wondering: was it doing the right thing and not telling me, or has the recipe changed?
That should still be correct, but try doing it from the top level of your source tree rather than in the directory of the file. Alternatively try using the --ignore-ancestry flag to skip the warning about the mergeinfo (which shouldn't be a problem in your case unless someone happened to have created a branch from the BAD_REV revision).