How to purge metric values from dropped metric extension in EM13c?

In EM13cR2, I had a metric extension (ME) that collected roughly 28M values. As it was causing major performance issues in EM, I undeployed it from all targets, exported the ME, and dropped it. That's fine, except all of the collected values were not also dropped, but cannot now be easily reported on, either. That's fine, as I don't need them, but how does one go about purging those values? As the collected values are strings, they take up the majority of my EM repository, and likely will for the next two years.

Searching MOS, but I haven't found anything yet. Anyone?