Setting for disabling SDI defense?

I found a way to make the SDI defense useless. This is not the proper way because SDI defense can still be researched and built, so this solution does not really satisfy one of the requirements of the question:

... prevent the building of SDI defense ...

Method:

1. Open the effects file for the ruleset you are playing (for the classic ruleset, the file is data/classic/effects.ruleset).

2. SDI defense has a "nuclear-proof" effect because of an entry in the effects file, so that entry must be removed or commented-out. In the effects file for the classic ruleset in Freeciv 2.5, the "nuclear-proof" effect is caused by the following lines:

   [effect_sdi_defense]
   type    = "Nuke_Proof"
   value   = 100
   reqs    =
       { "type", "name", "range"
         "Building", "SDI Defense", "City"
       }
   

   So to make SDI defense useless, comment out those lines by inserting semicolons like this:

   ;[effect_sdi_defense]
   ;type    = "Nuke_Proof"
   ;value  = 100
   ;reqs   =
   ;    { "type", "name", "range"
   ;      "Building", "SDI Defense", "City"
   ;    }