Purging JAMS history should be completely automated.  When JAMS is initially installed, a job named JAMS_PURGE is created which is scheduled to run every Sunday.  This job purges the JAMS History and then does a CONVERT/RECLAIM on the HIST.DAT file to reclaim space from the deleted records.

You should verify that the JAMS_PURGE job is successfully running.

If JAMS_PURGE is running, it could be that you're keeping more history than you want.  You can adjust the default values for the parameters to JAMS_PURGE to change the amount of history you keep.

It could also be that the HIST.DAT file grew so large because of an unusual event and the file is mostly free space.  The JAMS_PURGE job won't shrink the physical size of the file.  To shrink the file while preserving the history, do this:

$ MCR JAMS_EXE:JAMS_MASTER STOP SCHEDULE/NORESTART
$ SET DEFAULT JAMS_DATA:
$ ANALYZE/RMS/FDL/OUTPUT=HISTOPT.FDL HIST.DAT
$ EDIT/FDL/NOINTERACTIVE/ANALYSIS=HISTOPT.FDL HISTOPT.FDL
$ CONVERT/FDL=HISTOPT.FDL/STATS/NOSORT HIST.DAT HIST.DAT
$ PURGE HIST.DAT
$ MCR JAMS_EXE:JAMS_MASTER START SCHEDULE

Note that this make take a LONG time.

If you don't really care about the history and you just want to quickly shrink the file, do this:

$ MCR JAMS_EXE:JAMS_MASTER STOP SCHEDULE/NORESTART
$ SET DEFAULT JAMS_DATA:
$ DELETE HIST.DAT;*
$ CREATE/FDL=HIST.FDL HIST.DAT
$ MCR JAMS_EXE:JAMS_MASTER START SCHEDULE

This is quick but, IT WILL DELETE ALL OF YOUR HISTORY.