Is there a way to configure Exim to reject mail to a certain local host?

No, only to certain domains. To reject at SMTP time, you can put a line like this in your ACL:

deny message = this domain is deliberately rejected
     domains = a.certain.domain

To fail addresses in messages that do not arrive over SMTP, you can set up a router like this:

reject_a_certain_domain:
  driver = redirect
  domains = a.certain.domain
  allow_fail
  data = :fail: this domain is deliberately rejected