Most only allow you to enter specific web sites for blocking, rather than basing it off keywords, which means you'd need to enter a few thousand addresses to make a difference. You should look at a software solution instead, unless they have their own machines they are bringing and connecting with. If that's the case, then I would recommend limiting how much the router is available. Some models offer a schedule feature so you can turn the internet off after say 10pm so the guests can't be mischievous over night or whenever you're worried about it happening.