Since stopping it from loading the DLL isn't usually a good idea, perhaps you just want to stop a certain function to be called? you can do this easily with OllyDbg by pressing CTRL+N until you see the "Names" window...then scroll to the API you want to stop, and right-click, then go to "References". This will show you a list of all the locations where the API is being called.

Now you must make sure that you not only NOP the call, but also all the "push" parameters. Once that is done, you can safely continue execution (most of the time however, stuff will crash or fail because the app is depending on the API call).

Best regards,
Alex Ionescu
http://www.relsoft.net