Unfortunately, we've run into this one multiple times. Learning as we go, we've had to update policies, verbiage on Guest communications, etc. in order to avoid situations like this in the future. Yes, if they book online, it's very easy to prove that they agreed to the policies (cancellation policies included), as they cannot make a reservation without doing so. But you shouldn't have to be afraid of taking reservations over the phone! This is where you can do a lot of good by drumming up your place, giving restaurant or activiy suggestions, upselling some add-ons and packages, etc. This is GOLD! And they're already pleased with the service before they arrive. So how to avoid policy ignorance?
Couple of steps: 1) Always repeat the email address back to them using the phonetic alphabet (or one that makes sense to you and the customer alike) so that they are sure to get the confirmation once it's sent. 2) Put some verbiage in the confirmation email, the same one that goes to each and every customer, about agreeing to your policies and where to view them (not rubbing it in their face, but mentioning it on the sly so that if they were really THAT concerned, they could look them up). We end our sentence by adding in all the other wonderful things you can view on the website, so it immediately takes them from an uncomfortable subject to something that piques their interest. 3) Always, always, always copy yourself on the confirmation email. This is your proof that it was sent, when it was sent, and to what email address. Yes, many reservation systems will keep a copy of all correspondence to your Guests sent through the system, BUT there's not really an easy way to show this information to anyone without letting them in on the backside.
This has saved us a lot of headache, especially with folks who, for instance, booked the 26th thinking that it was a Saturday... and that's somehow our fault because they didn't read the confirmation email to check for accuracy? I don't think so! We all have our policies for a reason, so stand your ground and fight the good fight against those that would just as soon take advantage as look at you!.