Issue the K Q command to clear all the buffers or issue the
RCS RDISPLAY SESSION DETAIL USER=ALL
command. This will detail all the users so it might help you to see who is running up the buffers.
If you find a specific culprit then you can determine what is going on with that user.
If nothing else, when the buffer message is issued and you have OPS setup, it can automatically issue the K Q command and clear the buffers.
These are operational fixes.
You can adjust the collection values once you find a user that is collecting a lot of messages that may well be a hold over from the old days.
The RCOLLECT option can assist in this to be surgical in nature.
See the Reference Guide section 2-64 for examples but if you just do finds on RCOLLECT then you will see all the information available.