Created db migration to generate the field:

class AddEnabledToHosts < ActiveRecord::Migration
def self.up
add_column :hosts, :enabled, :boolean, :default => 1
end

def self.down
    remove_column :hosts, :enabled
  end
end

Now working on report code to detect that enabled = false. Enabled = true results in message being sent through unchanged.

looks good, a few comments:

1. consider using git, it makes this patching stuff much easier :)
2. I would think that hosts that are disabled, should not appear in the dashboard - do you agree?
3. it should be possible to toggle a host (enable / disable) - maybe a minor change in the host edit
4. maybe it should also be considered to enable / disable multiple hosts at the same time (can be a separate feature request)
5. filter hosts based on status (e.g. show all hosts which are disabled?)