On second thought. What are you actually proposing here?

I would suggest that there should be a purge method that actually deletes the contents of an entire index as opposed to "just" the managed entities.

Which index do you want to delete and do you want to really delete a single index or all indexes of a given application (in the file based directory provider case all indexes under the index base dir)?

Since in your case the entity is not managed anymore and the class probably does not exist anymore FullTextSession's

void purgeAll(Class<T> entityType);

won't work.
The alternative would be:

void purge(String indexName);

Is that what you are after?

Maybe this functionality should be added to IndexManager instead?