Keep resolving, but instead of resolving multiple A records, resolve just one. Fuck that one server in particular.
Delay updates by 1 week.
Answer randomly with the up-to-date or the out-of-date records.
Then you're harming your own customers/users as much as anything. They probably don't want to do that.
Resolve to porn.
Randomly resolve some requests with 127.0.0.1