Blocking IP addressing would kill hundreds (thousands) of valid sites in order to remove one violating website.
The fact you and SOPA/PIPA even suggested blocking IP addresses, rather than DNS addresses suggest that you & the creators don't necessarily understand how the internet works.
Here's a 30 second run down of how the internet works now:
I want a torrent website - I apply to Amazon.com for a virtualised/cloud server, which is shared by tens/hundreds of thousands of legitimate websites. PIPA/SOPA allows an interested party to block amazons IP address.
Tens/Hundreds of thousands of legitimate websites are now down duee to PIPA/SOPA.
I have a cached copy of my website from a few hours ago ... I apply to another cloud service provider, upload my server, and am operating again in a few hours.
Tens/Hundreds of Thousands of legitimate websites are left in turmoil wondering wtf is going on & why they've been SOPA/PIPA'd, most of them haven't done a recent backup, most of them have customers attempting to submit orders and are losing hundreds/thousands of dollars in sales by the minute.
RIAA/MPAA content is just so small compared to the rest of the world. It doesn't make economic sense to cause hundreds of millions of dollars worth of damage to the economy to chase a few millions dollars in possibly lost sales in content - that can't be proven to be lost sales, and academics, independent studies and common sense argue is actually provides a beneficial effect on sales anyway.