In this situation, you can use Amendment to Listing (TAR 1404). A provision in the amendment states that the seller is instructing the broker to cease marketing the property until further notice or until a specific date. The provision states that the listing is not terminated and remains in full effect.
If the seller is contemplating signing a listing with another broker, the seller will likely not agree to sign the amendment and this could lead to further discussions. If you determine that you wish to terminate the listing agreement, you can use Termination of Listing (TAR 1410). This form provides for early termination of a listing and determines whether the broker will receive compensation for early termination.