Right now i am receiving advertisements which are non-related. I changed my metatags so they are more myspace related but the ads won't change. How can i block an ad so new ones come?
It just takes time. don't bother blocking them, but after the mediapartners bots for AdSense look at your site, the ads will become better contextuallized and you will see better ads. (unless your site is about myspace, of course)
You can use section targeting to get relateds ads, see here : https://www.google.com/adsense/support/bin/answer.py?answer=23168&topic=8441
You can block an ad using competitive ad filter, to know more about it, visit the following URL https://www.google.com/adsense/support/bin/static.py?page=adfilter.html