This is one of my biggest pet peeves with Microsoft Word (all versions). I like the red squiggly lines for the spelling, but the grammer check drives me nuts and there doesn't appear to be a way to set them to off by default for new documents. You can manually turn them off for individual documents by going to Preferences > Spelling and Grammer (as mentioned above) and then that will stick when you save that document, but it will always be on by default for others until you do this again.