If needed you can also purge all (historic) data for a filer from the database, but make sure you have a (recent) backup of your database in case something goes wrong.
* write down the host Id's to be removed.
dfm host list -a
* stop dfm and start with only the sql service
dfm service stop
dfm service start sql
dfm service list - only sql should be started
* delete the host, this can take a while
dfm host delete -f <database_ID>
* start dfm again
dfm service start
dfm service list