Modem... not really, it wouldn't know the difference between facebook and the BBC. Some firewalls do operate in "layer 2" this a home or work connection? If the modem is a firewall, then blocking is depent on the model.
Some ISP's (mainly mobile) in the UK used to block facebook as they called it "adult content" and you had to agree to access it. To be honest need to know more about the connection.
EDIT - easy way to block it that kinda awkward for a normal user is to change the etc/host file and point the URL somewhere random.
That's kinda the problem corperate firewalls run up against. One of the current ways that is approached now is bandwidth limiting rather than blocking. Effectively you can get the page, but it takes 10 minutes to load. Stops people using proxies which can cause more problems that letting people having full access. Unfortunately that's something your average home box doesn't have.
Out of interest, when you use a proxy server the DNS request is sourced on the proxy rather than the client machine right? I think that's how squid does it... maybe...
She can go cruising for cock all she wants, just as long as the cooking, cleaning, washing and general family interaction etc. gets done first. I dont really give a schitt. Maybe if she spent less time on Facebook then she might actually find some cock maybe she could getb lucky and find one with a Prince Albert.