[exim] Reject ISP's hostname and use only my own domain

I searched around for a while but can't quite seem to figure this
configuration out.

I have my own FQDN that I use for my servers. My ISP also has a FQDN
preassigned for my static IP (of the form IP-octets.isp.com). I want to
configure Exim to accept mail that is addressed to user@mydomain and
reject anything sent to user@ISP_FQDN.

Everything I find online talks about rejecting sender hostnames when in
this case I want to reject one of the two recipient hostnames.

This is mainly for spam control. Right now it's not critical because
most of the sending systems are performing a HELO without an FQDN but
I've seen a few attempting to send to my system by using the ISP's
global naming convention. I just want to kill that ability ahead of
time before it gets abused.

In my config file, local_domains is configured only for my personal
FQDN, I don't have the ISP FQDN listed. I'm thinking that should
already reject anything addressed to user@ISP_FQDN but I want to be sure.

Thanks.