Very simple.

In CommandSet.ini , there are all the command sets for all things in the game. Command Sets are where you have the buttens on the menu. Making something unbuildable is as simple as copying a command set, renaming it, and removing the part you don't want them to build. Then, you just give that building/dozer the new command set. Example:
CommandSet GLAArmsDealerCommandSet
1 = Command_ConstructGLATankScorpion
2 = Command_ConstructGLAVehicleTechnical
3 = Command_ConstructGLAVehicleRadarVan
4 = Command_ConstructGLAVehicleQuadCannon
5 = Command_ConstructGLAVehicleToxinTruck
6 = Command_ConstructGLAVehicleRocketBuggy
7 = Command_ConstructGLATankMarauder
8 = Command_ConstructGLAVehicleBombTruck
9 = Command_ConstructGLAVehicleScudLauncher
10 = Command_UpgradeGLAScorpionRocket
11 = Command_ConstructGLAVehicleCombatBike
12 = Command_ConstructGLAVehicleBattleBus
13 = Command_SetRallyPoint
14 = Command_Sell
End
Now, copy it & rename it:
CommandSet NEWGLAGENERAL_GLAArmsDealerCommandSet
1 = Command_ConstructGLATankScorpion
2 = Command_ConstructGLAVehicleTechnical
3 = Command_ConstructGLAVehicleRadarVan
4 = Command_ConstructGLAVehicleQuadCannon
5 = Command_ConstructGLAVehicleToxinTruck
6 = Command_ConstructGLAVehicleRocketBuggy
7 = Command_ConstructGLATankMarauder
8 = Command_ConstructGLAVehicleBombTruck
9 = Command_ConstructGLAVehicleScudLauncher
10 = Command_UpgradeGLAScorpionRocket
11 = Command_ConstructGLAVehicleCombatBike
12 = Command_ConstructGLAVehicleBattleBus
13 = Command_SetRallyPoint
14 = Command_Sell
End
Make your changes:
Now, copy it & rename it:
CommandSet NEWGLAGENERAL_GLAArmsDealerCommandSet
1 = Command_ConstructGLATankScorpion
2 = Command_ConstructGLAVehicleTechnical
3 = Command_ConstructGLAVehicleRadarVan
4 = Command_ConstructGLAVehicleQuadCannon
5 = Command_ConstructGLAVehicleToxinTruck
6 = Command_ConstructGLATankMarauder
13 = Command_SetRallyPoint
14 = Command_Sell
End
Then, in the building's code:
***ART CODE***
; ***DESIGN parameters ***
DisplayName = OBJECT:ArmsDealer
Side = GLA
EditorSorting = STRUCTURE
Prerequisites
Object = GLASupplyStash
End
BuildCost = 2500
BuildTime = 15.0 ; in seconds
EnergyProduction = 0
CommandSet = GLAArmsDealerCommandSet
VisionRange = 200.0 ; Shroud clearing distance
ShroudClearingRange = 200
***BUILDING CODE CONTINUES***
Just ask if you have trouble.
