How can I block internet access?

Okay, here's the situation. We have three computers in our house which are all on broadband, through one linksys router.
My youngest sister is getting older and spending more time on the net. I know as well as you do that there's a ton of smut out there just waiting to be stumbled upon. Is there anyway I can block internet access on her computer through the router or the router's web based utility page? If not, is there any way to do it without bluntly forbidding her to use the computer? I don't wanna be a prick, but I need to protect her from this stuff.

How about educating her about how to avoid the smut pages instead of just denying her access to everything including all the USEFUL sites on the web? Make her use Firefox and disable ads with Adblock, and she should be relatively safe from UNWANTED smut.

Find some blacklist files (google for them) and put them in the local 'hosts' file on the machine you want to protect.

Something like
www.badsite.com 127.0.0.1