Turn off your internet.
Although your login might expire after 30 days.
Or you could link your Steam account to your YoyoGames account on their website and download the Stand Alone version.
Keep your fingers crossed that no forced updates happen during your development period.
What are you planned target platforms. If it's PC you will have less problems. Other platforms, especially mobile update their APIs frequently, so you'd need to update GMS 2 anyway to be able to deploy.