Rinse meat to remove residue and pat dry with paper towels. Prepare a rub by combining cayenne, paprika, chili powder, fajita seasoning, and Smokey Rotisserie seasoning in a small bowl. Squeeze lemon juice into bowl to make a paste and rub over entire surface of prime rib.
Follow manufacturer's instructions for the smoker. General rule of thumb is to maintain a temperature range of 255 to 300 degrees F (indirect heat), and smoke for 1 to 1 1/2 hours per pound of meat.
Beat heavy cream with an electric beater until stiff peaks form. Squeeze in lemon juice, 1/2 of lemon at a time, and beat in until incorporated. Then gradually beat in horseradish and salt and pepper. Chill for about an hour before serving.
Slice down prime rib and serve with horseradish cream.