So I would like to block a specific website on OS X and iOS devices and display a message in its place something like:
How can I do this so that the block works across all web browsers?
For OSX devices only: Use the hosts file (/etc/hosts) to redirect those hostnames to a local webserver.
Configure your local webserver to only serve up that message. Info here: http://osxdaily.com/2012/09/02/start-apache-web-server-mac-os-x/
For all hosts on a network:
Use an internal DNS server setup to be authorative for your domains and point them to a internal webserver showing your message.
You'll need your DHCP server to distribute your internal DNS server IP so it's used on all your client machines.
This is a great task for a DNS provider that can apply rules or custom white / black list for clients.
I’d start with one like OpenDNS which has been around for several years with free personal tiers and you can test this first without spending money.
Once you get a feeling if DNS works for your needs, you can then decide to pay someone to set it up and run your DNS or spend your time setting up your own DNS server so you can craft the rules you want using open source tools and hardware you own or rent.