Under BlackBerry Hub settings -> Display and Actions, on the bottom I set delete on handheld and server. It deletes everything - but the downside is if you delete something by accident, there's practically no way of restoring it, as far as I know. Also, delete on handheld and server will be set for all accounts: Facebook, emails, apps, etc.
yeah there is a bit of bugs with the fb & the Hub integration here. what you can do as the workaround for now:
- go to Settings
- Accounts
- on Facebook, toggle the "sync messages" from ON to OFF then SAVE
- let it loads, once finished, go back to toggle the "sync messages" to ON and SAVE
repeat those steps when you are caught up with the undeleted FB messages.