How do I direct a specific email address going to a virtually hosted domain to null?

You will need to create an alias (if one doesn't already exist) in the etc/aliases file that sends all received mail to /dev/null. Something like:

I just use "dev-null" as an example alias name. The alias can be named whatever you want.

Then in etc/virtmaps you can set up any address to map to the "dev-null" (or whatever you named it) alias:

Don't forget to run vnewaliases and vnewvirtmaps when finished.