We've had issues with Google and we're a peered network backbone. Literally took knowing someone to get them to fix DOSing our customer with one of their servers not liking our DNS entries. Never found out what happened either.
Sounds like a solid reason to null route the offending netblock or depeer entirely. If Google is hosting an abusive user/users and isn't willing to remediate the issue, there is no reason to glad hand them.
Unfortunately this was Google DNS itself munging DNS requests on a few of their servers. Not much we could do about it.