Yes. While Article 3 of the Code of Ethics requires that REALTORS® cooperate with other brokers, there is an exception when cooperation is not in the client's best interest. A seller’s demand not to cooperate with a firm or agent is a legitimate instruction regarding the sale of her property, and could be viewed as meeting this exception. Be sure, however, that the decision not to cooperate originates from the owner and not you.
It’s a good idea to get this instruction in writing from the seller. Should a broker or agent from that particular firm call for a showing, you can inform them of the seller’s decision to restrict cooperation on this particular sale and follow up by sending written confirmation.