Customer service won't take you off the mailing list? If they provided a prepaid return envelope, send it back. Oversize/weight mail costs extra.

Additional tips and notes:

-Always try unsubscribing through traditional means first, like calling the 1-800 number.

-Refrain from name-calling and profanity. The goal is to attract attention, not be an ass.

-Include the letter they sent you in the envelope. This ensures that if there's an account number or other identifier, they have it.

-Write your return mailing address on the envelope. If they don't open it, they can still see who you are and stop sending you junk.

-Laminate or tape the envelope, especially on the edges. This ensures it won't rip open and be undelivered.

-Prepaid return envelopes are billed to the account of origin when they are used. Bulky and/or overweight mail cannot be automatically sorted by machines and costs the business more money than a standard letter.

-If bulking up the envelope, be reasonable. Packing peanuts, pieces of cardboard, and water bottle caps are cool. Toenail clippings, salad dressing, and used Q-Tips are not. Again, don't be an ass.

An example can be seen here.