Hi,
this error only occurs if the site is redirected, because it’s forbidden. If the users wants to open sites which are allowed, it’s no problem. So they haven’t to ignore the ssl certificate error, they have to leave the site.
Sorry that I can’t give you another solution, but there isn’t one except the solution with IP-Adresses.
PS: I’ve had the same question a time ago, for reading the reason for that behaviour please have a look at: