Hi
First of all, what software version are you running on your IAP's?
I'm asking because on version 6.5.4.12 that i have you have an option in the firewall to add custom blocked URL's.
I have attached a screenshot.
Basically, you edit the wireless network, go to access, raise the level from unrestricted to network-based and the add a new rule.
If you have multiple sites (both http and https) that you need to block you have to add a rule for each one.
More usefull information you can find in the Instant Training, secion 4:
https://community.arubanetworks.com/t5/Controllerless-Networks/Instant-Training/td-p/474947
Best regards
Alex