The beaming rules are implemented in scm/auto-beam.scm.
I've identified a means of preventing the beaming in the second half of the
measure, but not yet for the first half.
As a temporary workaround, by overriding beamExceptions to '(), you can
eliminate the half-measure beaming. You will also eliminate whole-measure
beaming on eighth notes. At the present time I know of no way to enable
whole-measure beaming but disable half-measure beaming.