I'd say the first step is to seal it if you can. The issue is fixing the problem without scaring off wildlife with scent etc. You may want to buy some hornet traps were it has an attractant and the fly into it but aren't able to escape. Like this:
http://www.amazon.com/Rescue-YJTR-DT...QK6Y89FE6D1EAN
If that clears it enough, than seal it up with silicone to help keep others out. Or put up the traps, then in a couple days spray it with wasp/bug killer, and then seal it up as it should be pretty well cleared up at least temporarily.