Copies of Horse & Hound, telephone directories, bricks and empty envelopes were sent to the league after it launched an appeal for funds and equipment to monitor illegal hunting activity.
An email was circulated among pro-hunting individuals urging them to send empty envelopes or Christmas cards because a freepost address would mean the animal rights group would have to pay for postage.
The Countryside Alliance has denied any involvement. Spokeswoman Isobel Walsh said: "We are not supporting it and never have done."
Police are investigating whether the perpetrators could be charged with theft, fraud or sending malicious post.