A company sends a customer a free sample of its product of the month with an enclosed brochure stating that if the customer doesn't return the stamped self-enclosed "rejection" notice, the customer will be obligated to accept and pay for each successive shipment of the product for the next eleven months. Does the company have the right to sue the customer?

Sounds like fraudulent use of the U.S. Postal Service.

Receipt of Unsolicited Merchandise

You might want to contact U.S. Postal Service for verification/complaint.

Fraud - Prevention DVDs

You did not state where this is, but in the US no, there can be no assumed contract, and this would be fraud on several levels. It needs to be reported to the Attorney General, and the Post Master

Samples or products sent without any ordering are considered free gifts,

Now do they have a right so sue, heck yes, anyone can sue anybody in America, but I doubt they have much of a case

* but of course this is opinoin, please hire an attorney for professional legal advice.

If they send you a 48 inch plasma TV unsolicited by you valued at say $5,000.00 it's my opinion you can keep it. Check with a US Postal Service postal inspector, see what they say.