Inspect the magazine for a small white label printed on it. The label typically has your name, address and a 1-800 number listed. If not, look on the inside of the magazine for a 1-800 number.
Call the phone number, and keep the magazine nearby. The customer service representative you talk to may need information from the label. Tell the representative you want to cancel the subscription.
Remove the front cover from the magazine, if you do not want to talk to someone over the phone.
Insert the cover along with a note of instructions into an envelope. Seal the envelope, and address it to the company mailing out the unwanted material. Mail it, and eventually the company stops sending you magazines.