Your client should put the blocking in place that relieves
you from any problems legal or other
Is the sme box a gateway ?
What does the client use to get to the internet ?
win xp
win 98
fedora
lindows what ?
I believe it would be better to let the client put some blocking software on their own computer - (previous isp talking)
If you block access for one you block for all
Is the sme box at clients office or home ?
it will not matter then if blocking is put in place
http://www.squidguard.org/ has alot of info on it's use
and will do what you need