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It's one thing to link to a site that Google deems to be a spammer or otherwise unworthy site. To deep link to them many times is the kiss of death.
So unless I hear other details, this would be my leading theory.
I don't know why the site I have linked to is a PR0. They have no reason to spam as they don't sell on the web directly. Their database is only used by the trade. If I presume you are correct that I am linking to a "bad site" is it enough to stop the Googlebot following the links like I have already done now, and do you know whether it would be a permanent penalty?
Before you do anything else, you need to better understand the problem. For example, do other sites that link to this supplier have PR0 on all their pages? If not, do they use the "noindex" metatag on pages that link to this supplier?
Have you done anything else, such as engage in any link exchange programs? Did you use any SEO software that might abuse Google?
There are other things you need to explore first.