On one of my blogs I noticed that there is an ad which comes up on my adsense block that is totally unrelated (pretty much) to my blog. Is there a way I can make it so this ad does not show up in the future. simply because it is a waste of space which could be used for more specific ads. Thanks .
You may block the site by using AdSense's competitive ad filter. Google has an extensive guide on how to use the filter here.
Try also using section targeting for more relevant ads and block that specific one out by using competitive ad filter.
Use the competitive ad filter, you would first need to find out the URL of the site you wish to block. Google has a preview feature available for MSIE so you can do this easily.