1. Not really. Bits and bytes are bits and bytes. Your guests must sign off that they are responsible for their actions.
2. If you have the right router you can use OPENDNS. Not to explain fully but Netgear routers can use that and you can set the bar very low and lock out a lot of iffy content.
3. Even with that, it's not a sure thing.
4. If you want to assure that they can't do that, you need to shut down the wifi and internet.
I run a B&B and offer free wifi. I want to ensure guests don't access or download anything offensive or illegal. Is there a way I can stop this?