  Spectropolarimetric surveys reveal that 8-10\% of OBA stars harbor
large-scale magnetic fields, but thus far no such fields have been detected in
any classical Be stars. Motivated by this, we present here MHD simulations for
how a pre-existing Keplerian disc -- like that inferred to form from decretion
of material from rapidly rotating Be stars -- can be disrupted by a
rotation-aligned stellar dipole field. For characteristic stellar and disc
parameters of a near-critically rotating B2e star, we find that a polar surface
field strength of just 10 G can significantly disrupt the disc, while a field
of 100 G, near the observational upper limit inferred for most Be stars,
completely destroys the disc over just a few days. Our parameter study shows
that the efficacy of this magnetic disruption of a disc scales with the
characteristic plasma beta {(defined as the ratio between thermal and magnetic
pressure)} in the disc, but is surprisingly insensitive to other variations,
e.g. in stellar rotation speed, or the mass loss rate of the star's radiatively
driven wind. The disc disruption seen here for even a modest field strength
suggests that the presumed formation of such Be discs by decretion of material
from the star would likely be strongly inhibited by such fields; this provides
an attractive explanation for why no large-scale fields are detected from such
Be stars.
