Your question is a bit vague. If you mean that you are working with an agent to find property and now you want to end that relationship then a letter should be fine. If you have entered a contract to buy a home, sell your home, or entered into a buyer's representative contract then you probably, need a real reason. Most contracts spell out how to cancel them early if needed. Most service contracts state that the agent has to use due diligence in the performance of the contract. This is usually where agents leave themselves open to being fired. They stop trying to actively sell your home, or stop trying to find you a home etc.