Welcome to the Q&A Forum

If it were a simple toxic website that linked to your website you could have used the google disavow tool
http://googlewebmastercentral.blogspot.co.uk/2012/10/a-new-tool-to-disavow-links.html

But I think because it is actual spammy comments that it may be more difficult, google say to use this tool as sparingly as possible etc, first to contact the webmaster and as a last resort use this tool

I'd agree with Paul on trying to just create some great content, and offset the bad with good inbound links.

I think the number isn't important, if you have 1000 and every single one points to a relevant point, close to what they were trying to find- then there is absolutely no problem!

Just watch they don't all go to the homepage or are trying to use them to an SEO advantage etc; thats not what they're for and you will get penalised for that.

I've just done about 100 on a client's store recently and after a couple of weeks their rankings improved incredibly! they were getting hit with so many 404s etc

I also forgot to mention, I'd heavily assume a high number would be ok, because of instances like working on a store that contains 1000's of products and if a URL rewrite or something similar happens, all these need to be connected to the right place!