Stopping name server abuse

[Alex]

Hi,
We had a former customer who parked about 300 domains with his
registry on our server but is no longer a customer and hasn't moved
his domains. There aren't any hosts behind the domains.

Is there anything more I can do to block/prevent them from continually
querying my system outside of just redirecting them to localhost or
something?

It's not a terrible amount of traffic, but it's pretty substantial.

Unfortunately asking him nicely didn't work.