Scrub it off with a fungicide solution (decorating secion of DIY / hardware shop) - let it dry thoroughly and scrub it again.
Then decorate when it's had a chance to fully dry out.
It will very probably (almost certainly) be caused by condensation. Warm wet air from the house in contact with the cold surfaces in the porch. You may never stop it competely but there are some things you can do.
A dehumnidifier in the hall will certainly help but there's a cost associated - in the UK Argos do one for £40 - I paid £160 for mine from EBAC - it can easily suck up half a litre a day from the damp air in my house.
If the porch has an inside door then keep it closed as much as you can.
When cooking and bathing keep the doors to kitchen and bathroom closed and open a window in those rooms to let the steamy, damp air out.
Ventilate the porch to allow the escape of any moist air that does get in.
If the porch area has no natural ventilation consider providing some, or, if you can safely leave the outside door open for short periods during the day do that as often as you can.
Good luck...
Peccavi, January 2009
