If you didn't find it already, disable auto updates on the properties page / updates tab. Select "Only update this game when I launch it."
But won't that still update the game when you launch it?
No if you install the patch 6 branch, you stay on THAT branch and no updates on the "normal" branch will be loaded. You'll have to explicitly choose the main branch again when you're ready to upgrade, after which it will downlad patch 7 and subsequent hotfixes.
