Be careful, the messages you delete will probably be deleted on your mail server, too.
I think you can remove the oldest messages for Exchange ActiveSync accounts. Those the accounts with the option named "Message Sync Timeframe." If you reduce this sync timeframe, I think the oldest messages will be deleted on the PB (but not on the mail servr).
This is one of the features that hasn't yet made it to the PlayBook but most certainly will as the platform develops towards BB10. I am also waiting on the feature to confirm delete off of email and playbook or just the playbook.