I heard that many comments in the blog cam make down pagerank. Is it correct? Than, how to reject incoming comments in wordpress blog without selecting ?
No, it's not correct. How on earth could Google possibly justify penalising bloggers for the actions of their commentators?
I use askimet to filter out comment spam. Other than that, I don't get a lot of useful comments so I don't sweat this issue. However, I don't buy the whole thing about it lowering page rank. I've seen the opposite...pages with a lot of traffic and interaction usually do well. Unless of course the comments being approved are trash.
Make all link in no-follow. It can prevent your blog form spamming or other non-thematic link which is bad for page rank.
Google would never downgrade your SERP/PR if you have more no of comments....But spam comments may degrade page's quality....Nofollow is the best option to avoid spam