Block websites for none proxied devices

A customer of us is using a Fortigate and would like to achieve the following. 

A visitor connects to their guest or office wifi with their own laptop or phone. The customer wants to blocks youtube access for example. The guest user when trying to visit is either redirected to a : Blocked by Fortigate or a customers own website. Is this possible? Please note that we are not able or allowed to install any certificates on the visitors device.