How do you shield a Center speaker?

Most manufacturers that shield their speakers use the bucking magnet approach, it's simple and effective. Any change in the Q of a driver is too small to make any difference and will not be noticed by your ears. IMO, lining the inside of a speaker cabinet with metal is bound to change the tonal characteristic of said speaker for the worse. Manufacturers strive to have inert cabinet interiors, not highly reflective surfaces.

Will sheilding taped to the outside have any effect? It seems the simplest (maybe not the prettiest) solution.

I have to agree with Merganser.

It's difficult to "shield" magnetic fields.

The bucking magnet solution is probably best. From outside
the speaker, the magnetic fields of the two magnets - the
bucking magnet and the one in the speaker driver - will
essentially cancel.

Inside, the voice coil is much closer to the driver magnet -
so it doesn't affect the driver much.

Putting a shield material around the speaker would be more
of a sonic degradation than any effect from the bucking
magnet. Additionally, one can't put shielding material
in front of the driver - so the speaker will leak magnetic
flux through the front.

Dr. Gregory Greenman
Physicist

I had this same problem once, brought the speaker in to a factory authorized repair shop. They said it was shielded and demonstrated to me on some computer monitors and tvs that the speaker wasn't the problem. They said it was most likely the amp in the receiver. I wish I had made notes, I can't remember what it was. I gave the reciever away.