The fastest way? Use the web API to find all the product keys you want to delete and then delete them:
https://wyday.com/limelm/help/api/limelm.pkey.advancedSearch/
https://wyday.com/limelm/help/api/limelm.pkey.delete/
The easiest way? Use the LimeLM interface to search for the product keys. Then select multiple at a time and delete them.