The simple cheap option is moving the sub farther from the set if it causes any picture distortion. It is not, in most cases, a permanent effect. However it could be if left very close for too long. The rainbows caused can be removed with a degaussing tool or, if the televison has a built in degausser.
Prices on lead sheets or other shielding materials can get quite pricey.
Your best bet is to put the sub in place, then slowly move the tv into position and watch for any changes on screen. A solid blue screen is easy to tell if there are any distortions.