Android automatically connects to all WiFi networks it has configured. So basically, if you do not want that to happen, you could either turn WiFi off (which would also disable to connect to the other WiFi networks, which is not what you want) -- or delete the configuration for the network you don't want to connect to automatically (which would make you lose its settings, on the first glance).
The first option is not in your interest -- but the second can be helpful, even if you don't see this immediately. You could use an app like WiFi QR Share first to convert that networks settings into a QR code, then print out this generated QR code. Store it in a safe place (e.g. laminate it, cut it down to credit-card size, and put it in your briefcase). Now delete this WiFi networks configuration -- it will no longer auto-connect.
If you want to connect it manually, that does not mean to enter all the configuration again: Simply open your QR Code reader (Barcode Scanner should do fine), scan the QR, and connect. Don't forget to delete the configuration again afterwards.
Unfortunately, there's no way to mark a network "manual only", so this seems the easiest way to me.