Called them up. Pulled out the DNR papers for my legal collection/possession of eastern garters.
The guy was unflinching.
... HOWEVER...
He said it's fine for me to keep the snakes, as long as they're indoors. So, I know the problem isn't so much a violation of city law as it is our neighbor trying to get back at us. :P
The man was very cooperative, almost sympathetic. I'm sure he understand that there are plenty of people in the city keeping "exotics." (The note said "It is prohibited to keep exotics. It is illegal to keep snakes.")
So, all I have to do is take the snakes out of their outdoor enclosure, and I'm good to go.
As far as our neighbor is concerned... I don't think I'll be provoking her anytime soon. She's like... 70 or something anyways, so she'll be dead soon enough. :P