Norton Internet Security can do that, I believe. Or have mom take the cable/DSL modem's power brick to work with her every day? :evil: Or... or... show her how to do a Run As on Control Panel > System to elevate to Admin level and disable the NIC in Device Manager? Slightly clumsy, but if they have broadband, then there ya go. A Limited-class user wouldn't be able to re-enable the NIC.
Or if she has a router, you could set up a port-blocking rule that blocks ports 1-65535, and she can just log into the router to turn the rule on and off. Again slightly clumsy, but unless the kids are creative enough to hard-reset the router or take it out completely, it would work to keep the sprouts off the pr0n sites until Mom gets home...