There are a variety of ways of curbing feedback when playing amplified, including...turning the volume down (amazing that it's not more obvious)...covering the soundhole...eq adjustments...alternate signal sources (magnetic, piezo, mics et al)...moving or directing away from speakers...but here's one thing not to do...

I once saw a guy's guitar literally explode because it couldn't endure the solution to feedback. This happened during a gig, in his hands, while he was playing. The amplified explosion stopped the band cold. Seems the player had, just before the gig, filled the guitar with expandable foam insulation, somehow sealing the soundhole. The glued seams of the flat-top couldn't withstand the pressure and let go with a bang. I don't remember what make of guitar it was, but I seem to remember that it was blamed for the accident. (As you've guessed, the guitar did sound like crap.)

The moral of the story is, GET PROFESSIONAL ADVICE, and don't do anything to your guitar that you can't undo.

Mooh.