10 days ago i went to dot au blog site
going here i commented only against one post.
but now i see more then 2000 backlink for my site
here note that: my site already have Google PR 5
this kinds of back link is harmful for my site
now what should i do?
Yes you don’t need to do anything leave it to Google. Google panda is now counting backlinks. Panda will automatically deleting back links if it is not related to your site or if your just doing spamming… 
If your site are already crawled by Google, you will notice changes in your backlinks…
Why do you think that these links will harm your site?
Apart from asking the blog owner to remove links to your site, I’m not sure what else you can do. Next time, think about whether you really want the backlinks from a forum before posting a link to your site!
The search engines may eventually drop most of the backlinks from the site. However, it may not hurt the site ranking.
What do you mean by: never do spam and why G. would ban your site?
fastreplies
The explanation for this is that, on each webpage for that particular blog, there is a section that lists the last 5 (or 10, or 20, or whatever) blog comments. As new blog comments are made, your blog comment will drop down these lists and eventually drop off. Google will then take maybe up to 90 days to drop these links from its data, leaving you with just one IBL from this blog which will be coming from your original comment.
You needn’t do anything. In Google next update, those backlinks will be deleted.