Nasty slap echo

For the best cure for this problem, check out Michael Green's "Room Tunes" products. They will remove the slap echo without overdamping the sound. Simple small pillows you can affix to the wall to ceiling area with pins or tape. A combination of absorbtion and diffusion. Don't underestimate these things by looking at their appearance, they really work right.

Streetdaddy, walk around the room clapping and you can pinpoint exactly the spot(s) where echo is concentrated. Treat this spot with something like the Room Tune pillows. You can make your own from fiberglass insulation and heavy duty aluminum foil as a temporary experiment. Adding treatment all over the place may be misguided and overdamp the room. Pinpoint the specific area and only treat there. Your "ping" is probably half way between corners at the wall/ceiling junction.