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When you see the wasps around, look at the exterior of the chimney and possibly from a ladder.
What you may see is that they are not coming down into the chimney through the top or chimney termination but in through small cracks in the brick where the mortor has come loose. I would spray the area for the wasps, wait a resonable amount of time and seal the crack and then purchase and install a http://www.woodlanddirect.com/Chimney/Top-Sealing-Chimney-Dampers/Chimalator-Top-Sealing-Chimney-Damper lock top type chimney cap.
My professional fireplace company has two options, of which we've implemented one for our clients in the past. If you want to essentially permanently block it off, there is something like an "air pillow" which will blow up inside the flue to block it.
The solution we've used is a spring-loaded chimney cap. It sits on top of your masonry chimney flue above the roof. It has a long chain which comes into your fireplace. You use the chain to release it when you want to burn a fire. You pull the chain down to seal it when you are not having a fire.
Added advantage: better energy efficiency and indoor air quality! Keeps air from blowing out the chimney or being sucked backward through the chimney into the house!
A few questions first.
Is the chimney functional? Is the flu venting anything in the home (i.e. furnace)? Does the chimney go through the roof, or next to the roof on the eave or rake?
If the chimney is not functional and it does not vent anything in the home, you may want to address the problem at the roof by closing it off. This will make sure no bugs or water can enter into the home. If the chimney is through the roof, you will need to tear the structure down below the sheathing, install blocking around the chimney box, and install new sheathing over top the chimney. Exterminators are always the best experts to call for bug problems. If they keep coming back, it may be time to try a different company. Hope this helps!
AROCON Roofing and Construction, LLC
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