Guilt by association - SEO - on a shared server

I'm sometimes called in by auto dealers to restore their website when some spammer or hacker ruins the IP address for everyone. Today this happened to me when I caught another auto dealer who apparently paid for back-links that ironically ended up on one of my other forums. Here's a sample of it:

Hi friends. There are some information which I should share with you about FJ.
The Toyota FJ Cruiser is a retro-style SUV produced by Toyota introduced as a concept at the 2003 Chicago Auto Show and in production form at the following North American International Auto Show. It went on sale in early 2006 as a 2007 model and features styling reminiscent of the original Toyota FJ40 Land Cruisers from the 1960s (particularly the front fascia, white roof and wrap around rear windows) and is based on the Land Cruiser Prado platform. The FJ Cruiser was not originally intended for production but due to enormous consumer response about the car following the North American International Auto Show, production eventually began in early 2006. FJ Cruisers are built by Hino Motors in Hamura, Japan.
.....y Toyota

Now the question is, should I alert the other 143 accounts that are on the server or just handle it internally?

Is Google going to punish all of them because there was a known spammer on that IP address? I suppose their SEO may end up suffering as well?

The abuse emails have already started coming in, and I fear, with good right, that the IP address may just end up black listed in whole or part.

Time for a new IP address?

I'd pick a new ip for the whole block, and consider buying an ip block 1 per account.