To disable unit:
1) go to the top menu named Scenario
2) in there, you will find a submenu named Forbidden Unit
3) open it -- you will see a gray window with a small, empty white space at the top
4) if you want to disable units of player2, type into that little box the number 2 and press Enter. (number 3 for Player3, etc.)
5) after you have done that, it will open up a list of units/buildings/technologies that you can disable. Click on them to disable.