Interestingly, giving negative bonuses to upgrades (in the format weapon_bladed bonus -1 for example) removes the unit with that weapon technology from being able to be recruited but is still listed as available in the relevant building scroll. A building that negates the negative bonus will restore the unit's availability for recruitment. Clued-up modders will see the potential for this feature :) (The Complete EDB Guide, under "Negative effects")
The above paragraph was added 19 Oct 2007 soon after the discovery of this.