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I noticed that svn propdel --recursive doesn't work if the properties are not set on files at the top level. If you want to delete all svn:mergeinfo from any subtree or file inside the branch, it's best to create a list of files on which to delete the property individually:

svn propget -R svn:mergeinfo --xml ./*
gives you output containing among others this list of files. If you're on unix, you can use grep and sed to massage the output so that all lines looking like
    path="path/to/some/file">
will be converted to
svn propdel svn:mergeinfo "path/to/some/file"
following line passes this output directly through bash so that it gets executed immediately:
svn propget -R svn:mergeinfo --xml ./*|grep ' path="'|sed -e 's/ path="/svn propdel svn:mergeinfo "/'|sed -e 's/>$//'|bash

Users of other platforms will have to use some advanced text editor, or install ports of grep and sed for their platform.

Before committing, it's best to verify you didn't miss any properties due to possible weirdness in the xml output, so run
svn propget -R svn:mergeinfo --xml ./*
again to double-check.