When we do our branching and merging with svn, we always have the same thing happen.
We get a latest copy of the trunk.
Create a branch and switch.
Read the detailed article at http://www.collab.net/community/subversion/articles/merge-info.html
It's long but worthwhile. Note in particular the suggestions in the "Parting Thoughts" section at the end, which include hints for reducing mergeinfo noise.
Also, instead of using svn propdel to clean up unwanted mergeinfo, consider using svn merge with the --record-only option.