Cancel RWB messages in bulk

Hi,

We have around 100,000 messages in System error status in RWB in our Dev system.

These message have accumulated because of channels were open and other un-monitored activities.

Due to this our heap memory is getting full, causing system to Crash.

I have stopped all the channels and trying to clear the message & setting them to cancel status from Message monitoring in RWB.

However each 1000 messages take huge amount of time to set the status to cancel.

Is there any other way to do mass deletion of thees RWB messages.

I don't see these message in SXMB_MON and RSXMB_CANCEL_MESSAGES as well.

Please advice.