I have small blog and when I count the density of keywords in that, unexpectedly keywords like "comments", "share", and "admin" comes up as keywords, which I don't want.
How can I exclude keywords from appearing in keyword density counts?
You can't exclude keywords from appearing in keyword density counts by editing your website's code.
Depending on the software you're using to do the keyword density check, you may have an option to blacklist words so they aren't counted on the keyword density report.
On a side note, if you're planning to rank your website on major search engines like Google, Bing, Yahoo, Yandex, etc., you will probably don't face any issues.
As closetnoc said before, Google and other major search engines doesn't use keyword density as a ranking factor, instead they use something called Latent Semantic Analysis (LSA). On this topic, I can recommend 2 readings for you: