That's going to complicate things, that 1803 wants to come in so quick to spoil your plans. Here is WUShowHide to stop it...
How to temporarily prevent a Windows Update from reinstalling in Windows 10
You must catch it trying to come in again. Could be you are too late, though, if it has begun to download. I guess let it do its work - it may be successful this time now that 1607 is in.
Good luck, & keep us informed.