Hide comments… from Search engines

Hi,

I have read somewhere that comments can hurt your keyword density and your ranking. I tried to install a show/hide comments plugin (“Hide or Show Comments”) but when I analyzed the keywords on the page, the keywords from the comments were still used.

Is there any way to get best of both worlds: been able to get lot of comments without them ruining keyword density? (You know what I mean: when someone starts talking about something not directely related to the subject…)

Thanks