You could try something like this:
# /db/seeds.rb
ModelName.class_eval { acts_as_mappable(:auto_geocode => false) }
# Create seed models...
ModelName.class_eval { acts_as_mappable(:auto_geocode => true) }
This might even work,
ModelName.acts_as_mappable(:auto_geocode => false)
EDIT:
Another solution would be the skip_callback
method:
ModelName.skip_callback(:validation, :before, :auto_geocode_address)