It is not really possible to prevent spammers from visiting your blog as long as it is active and online. Blocking IPs only works in specific cases but mostly spammers spin IPs on daily basis.
The best way is to secure your blog instead of wasting time blocking spammers. If you are serious about your online work, you must get your own hosting and domain name. Blogger is a free service for beginners to feel the water of the web and not a tool for building real websites and blocking spammers.
Hope that helps!