I think that I would first treat the affected area and stop the spread with a borate insecticide, like Timbor, it works best if no finish is on the wood so it can penetrate.
Post some pics so we can see the structure and the affected area. It could be that replacement would be the easiest and cheapest, saving diagnostics.
Powder post beetles like unseasoned wood or wet environments, so keeping moisture down can really help.
Here is a site with some good info that may be helpful:
http://www.ces.ncsu.edu/depts/ent/notes/Urban/ppb-wif.htm
