@Jonathan951 In my opinion, WiFi is not optional if you're renting through Airbnb, starting with the simple reason that Airbnb itself can only be accessed via the internet. If guests have a customer service issue, need to alter their booking, update their payment method, or send you a message that can be viewed by Airbnb staff in the event of a dispute, they need to have internet access during their stay.
Asking guests to leave the house and find one of those crappy hotspots does not cut it. That's a guarantee of bad reviews. The Guest Account idea that others suggested is worth looking into, especially if you have an old-fashioned internet plan with data transfer limits.
I don't want to make any assumptions about what content you want to prevent guests from browsing, but if it's not illegal, you might have to ask yourself if it's really any of your business...