@killmasta93:
You cant block https unless you use WPAD (works but may or may not block on android phones) or explicit proxy which means installing CERTS on each computer/devices.
I'm not 100% in line with this view:
WPAD is used to avoid manual proxy configuration on each and every device (Web Proxy Auto Discovery). Nothing more nor less than this.
On the other hand, your point about certificates to be installed on each device is rather linked (perhaps) to some willingness to implement MITM at proxy level in order be able to scan HTTPS flow content.
Aside the fact that this is weird :P this is different from access control, fully achievable without MITM.
Regarding MITM, I'm also surprised it's a matter of certificate installation. I would rather say that you have to trust, on each and every device, CA having signed certificate used by Squid. Either manually or deploying CA public key on each and every device ;)
Or I misunderstand your point :-[