How do you treat a cat allergy?

Avoid, meds, shots. The most important treatment for allergies is avoidance. If the cat is staying, then nasal steroid sprays are the most effective treatment but will likely not completely eliminate symptoms. Antihistamines like Claritin may provide partial relief. Allergy injections are the only long term treatment that will change your allergic response to cat dander--but it takes patience and time.
OTC. Cat allergies are treated the same as other environmental allergies. Use an antihistamine such as claritin, zyrtec, or allegra. There is also zaditor (ketotifen) available for eye symptoms.