Get From Marchenko to the Delegates Instantly If you choose not to, but then end up wanting to use it, you can still equip it in your inventory and use it much like a Multi-tool when pointed at the boss. Now, when you face Marchenko, you'll have the option to use the Killswitch before the fight ever starts. If you didn't get the killswitch earlier, look under the main desk with the laptop, and move the studio bags aside to find a briefcase containing a Pocket Secretary and the Killswitch.
Go past the catering room to find a lone guard at a desk, and take him down. If you missed this one, you have one more chance to find it in London's Apex Centre by the catering room where you find Miller. This reveals a small recess in one of the lockers that has Marchenko's killswitch - its about the size of a Multi-tool. They also mention meeting by the lockers, so head to the lockers in the top left corner, and use Smart Vision to locate a hidden button behind the lockers, which you can activate by jumping on top of them. One of the laptops has a message that references a "killswitch" for Marchenko called Damocles Sword. When in Mission 14: G.A.R.M., once you enter Hangar 2, look for the large tent in the top left corner that houses 2 broken box droids with three operatives standing around them. Trying to defeat the final boss of Deus Ex: Mankind Divided can be a lengthy one, and may cut into the time you need to try and save the delegates - but there is a way to cut the fight short.